It was meant to be just another England training session. Then Theo Walcott picked up the ball on the right wing...

He received the pass on the right, taking a precise touch to help the ball run across his body. The first defender sought to show him down the line, where there was cover, and he dropped a shoulder as if to follow, but then Theo Walcott twisted and came inside. He weaved past Jimmy Bullard and David Bentley but looked to be drifting too far across the midfield when he suddenly turned, spun and darted through a gap for goal. Paul Robinson came out to narrow the angle but Walcott drove the ball past him.

Training seemed to stop for a moment as the England players absorbed the quality of the goal. Some applauded. It was only a practice match, but all the same. As one player later told me: "It was the speed of his feet, he's just so quick with the ball."

On the sidelines the England coach Fabio Capello, who spends much of a session barking orders at players, was temporarily silenced. His decision merely to include Walcott in the squad for last month's World Cup qualifiers against Andorra and Croatia had not been universally anticipated or applauded. Walcott had begun the season so patchily for Arsenal, there was even doubt about what position he was playing.

The goal was the talk of that day's training, and soon leaked out to the press pack that covers England games. Even so, few anticipated that Walcott would be thrown in against Andorra on the Saturday. Capello's predecessor, Steve McClaren, had not picked him once, and the teenager's 21-minute substitute appearance, under Capello, in Trinidad in June, was his first England outing since his debut, again off the bench, in May 2006.

But the goal in training had changed everything. Capello, a cautious man by nature, decided to gamble. The next game would be his first competitive fixture as England coach and Walcott would make his first start while David Beckham would be left on the bench.

Still a teenager, but now brimming with belief, he tore Andorra apart in the opening 20 minutes in Barcelona. He was, said Capello afterwards, "incredible". But after that opening burst, in which he created a chance Wayne Rooney should have converted, Andorra's part-timers crowded him out. All the same the player was encouraged. "The confidence is there now," he said. "I've started every game this season for Arsenal, bar one, and I feel I've so much confidence. I'm training well and, when you are training well, your confidence flows. It is just instinct really. I feel great. I'm buzzing at the moment. I'm flying. I just cannot wait for Wednesday."

However few anticipated his making the starting 11 in Croatia four days later. Walcott had been impressive, but it was a critical fixture at a venue where England had been well-beaten in qualifying for Euro 2008, and where Croatia had never lost. The right-sided player was expected to have to do a lot of defensive work as Croatia's left-back, Daniel Panjic, was a converted winger whose attacking forays were a key part of Slaven Bilic's armoury. Surely Beckham, now rested from his trans-Atlantic flight, would be back.

Capello retained his faith in the future, Walcott played. It was immediately clear that the team had been given instructions to pass to him whenever the opportunity presented itself but Walcott's early touches were not encouraging. Then, after 26 minutes, Rio Ferdinand intercepted a poor goal-kick and stepped upfield. His surge prompted confusion in Croatia's defence and the ball broke to Walcott. Too often, in this situation, he would have shifted the responsibility and passed. Now, buoyed by his goal in training, he seized the moment. Having taken a touch to wrong-foot a defender he drove the ball inside Stipe Pletikosa's far post.

From then on, Walcott ran at the Croatian defence with gusto, drawing a cynical body-check from the lumbering Josip Simunic. Though winded he got up and punished Croatia with a second goal, drilled in after a neat passing move involving Rooney, Emile Heskey and Frank Lampard. With eight minutes left he ran on to Rooney's flick and sprinted away from the home defence to complete the first hat-trick of his senior career, for his country, at the age of 19. Going into the game many had expected England to lose as they had done a year earlier. The optimists suggested England might get away with a draw. Nobody foresaw a rout inspired by a teenager. A national hero was born.

"He is so young and so dangerous for defenders," Capello purred afterwards. "He is one of the most important young players in England. When he plays like this evening it is fantastic." England's captain John Terry was just as effusive. "When he's on the team-sheet any defender will be petrified. That performance showed what he's all about. He's got the pace and excitement that gives the fans a lift."

The teenager restricted himself to a few polite but anodyne comments about his own performance preferring, as ever, to focus on the success of the team. "We wanted to prove how good we are and if we keep playing like that I'm sure we will." His modest self-assurance following such a spellbinding display impressing not just Capello, but also his club manager Arsène Wenger.

It is early days but the initial indications are that Walcott will remain level-headed even though his hat-trick has elevated him to the status anticipated when he was first called up by England, prior to the 2006 World Cup. This afternoon Kazakhstan's defenders will be as wary of him as of Rooney, maybe more so given his pace. As Rooney himself said, referring to that goal in training: "I've not seen anyone as quick as him. He scored a goal in training the other day and everyone looked at how quick he was. When he gets going he's just unbelievable." In 2006, when Walcott broke into the England squad as a callow 17-year-old, the situation had been very different, however. His involvement at training was, said fellow players, "embarrassing". One squad member recalled: "He would try and beat players like Rio Ferdinand, Sol Campbell and John Terry with pace alone. They are too good for that. He would just bounce off and just kept going down. The first time we thought he had broken his leg. The second time it was, 'hang on a minute'. After that it was an embarrassment."

In his recently published autobiography Jamie Carragher was just as damning. "He was nowhere near ready," Carragher wrote. Sven Goran Eriksson, the England manager who had picked Walcott on the strength of what he had seen while attending Arsenal training sessions, insisted at the time: "I've not regretted picking him. The two goals he scored in training last week were very, very good. He's a huge talent, no doubts about that. He is mentally ready for it. He is here and he deserves to be here." Fine words, but Eriksson never backed them up by picking Walcott to play an actual match which suggest the players' recollection of his impact at training was more accurate.

Two years on Walcott had won his their respect. His goal, and the squad's reaction, must have had Rooney casting his mind back five years to another training session, this time in the Durham countryside, shortly before England were to play a vital Euro 2004 qualifier against Turkey at Sunderland. Rooney, then 17, had yet to start for England.

In the final training session Eriksson staged a full-scale practice match. Rooney went through for a one-on-one opportunity against Robinson. Rooney chipped the keeper with such sang-froid the rest of the squad broke into applause. Eriksson picked him to start and England defeated Turkey.

"There are times," recalls one of Capello's predecessors as England manager, Graham Taylor, "when something a player does in training makes you think he is on the boil, that this is his time. Of course, there have been occasions when you have felt like that and he has not done a thing. But you go with your gut feeling. You hope he takes it on to the pitch."

Once training consisted of lapping the outfield and running up and down the terraces. The ball was kept to match-days – or perhaps, Fridays at the more enlightened clubs – so players would be hungrier for it on match day. Training is now seen by most successful managers as just as important as match play. And if the manager takes training seriously, he is looking for players to follow suit, while recognising there will always be exceptions. Craig Brown, the former Scotland manager, was assistant to Sir Alex Ferguson during the 1986 World Cup. "I said to Alex, 'Willie Miller's not doing it in training'. Alex said, 'I've got him at Aberdeen [where he was then manager], trust me, he does it in games'. And you do see players who save themselves in training, they are getting a bit older, and they keep it for matches. You get to know who can be relied upon. If Steven Gerrard was not doing it in training you wouldn't drop him as you know what you will get in games."

Unless, of course, there was a reason. Glenn Hoddle, for example, knew he would have to omit Jamie Redknapp from England's 1998 World Cup squad following injury "when I could see his stride pattern had changed" while training. Then, added Brown, there are players who are world-beaters in training, but not in matches. "Dickson Etuhu, who I had at Preston, was fantastic in training, but not so good in matches. Roy Hodgson [Fulham's manager] rang me about Etuhu. I said, 'If you can get him to play for Fulham the way he does in training, and I think with your experience you can, you have got the next Patrick Vieira'."

Ally McCoist is regarded as a cheeky chappie but, said Brown, in reference to the shooting practice which often ends a session, and are the excuse for many players to simply blaze away, "McCoist always took training very seriously. He was very conscientious about his finishing. He never missed."

Which brings us back to Walcott. He's new school. Eager to learn and hard-working. "I'm still trying to improve every training session I have and that's all I can do," he said. Taking training seriously might just have altered the trajectory of his life.

Case study: Don Hutchison

Another to make a key contribution to training

Scotland, not for the first or last time, were short of strikers. Craig Brown had given Don Hutchison his first cap, as a substitute, a month earlier but, as the manager prepared for Scotland's April 1999 friendly away to Germany, he took another look at the then- Everton midfielder. "We had a practice session and we put him up front to have a look at him," recalls Brown, "we didn't tell him what we were thinking. He looked very good as a striker. It didn't make up my mind but it made the decision a bit easier. He scored in Bremen and we beat Germany 1-0, which was a very good result." Later that year, playing in attack, Hutchison scored against England at Wembley in the Euro 2000 play-off second leg bringing the Scots to within a goal of forcing extra-time.