TV review: Eddie Izzard's Mandela Marathons - Go, Eddie, go! But get some proper shoes

Eddie Izzard is a deeply unusual man. I don't mean the cross-dressing or the old line attributed to him, about being "a lesbian trapped inside a man's body". I'm referring to his almost pathological appetite for running marathons.

Celebrities undertaking acts of physical endurance are two-a-penny. Christine Bleakley in a wetsuit! David Walliams in goose fat! Izzard, though, ran 43 marathons in 51 days in 2009, a feat of such leg-boggling proportions that you couldn't help but think there was something a teensy bit more complex than fundraising for charity going on. Appropriately, the film that covered his seven-week epic was half psychodrama-in-trainers, half roving conceptual-art installation, with Izzard hauling his not exactly Paula Radcliffe-like figure about the hard shoulders of Britain's A-roads.

The result was utterly, grimly fascinating. He must have got a taste for the pain because he set about organising the project that culminated in his two-part documentary, Eddie Izzard's Mandela Marathons.

But if you were a fan of the previous programme – run, Eddie, run! – this one lacked what you might call its purist rigour. In honour of Nelson Mandela's 27 years of incarceration, Izzard attempted to run 27 marathons in 27 days, tracing a route through South Africa that resonates with the life of the great man: his birth-town, the site of his old school, and so on. In other words, as Izzard jogged "like a sack of potatoes with a mission", to use his own words, we were treated to a breezy biography of Mandela's early life.

More revealing though was the glimpse of Izzard's single-mindedness, a determination that verged on self-delusion. "I want to say thank you for existing to Nelson Mandela," he said, sincerely enough. But there were indications that Izzard was riding for a fall: his meagre training regime, sheepishly admitted to, and his insistence on running in shoes that made flip-flops look sturdy, on roads so rough that signs warned drivers to remove dentures and hearing aids.

Throughout – on the commentary, in gig clips, even as he's examining his own blood-clouded urine – he maintained that characteristically off-hand tone. The more casual he sounded, though, the more you sensed the granite will beneath. But Izzard himself is mere flesh, and there was an awful lot of veldt left to trek at the end of episode one.

There is a walking, talking rebuke to the practice of primogeniture, whereby the estate passes to the first-born son: its name is Jamie Blandford, the wayward heir to Blenheim Palace. He appeared, unwisely, in a previous programme of The Aristocrats (More4, Saturday **), a clip shown again in last night's profile of the Rothschilds, as a contrast to their own family arrangements. For them, famously, you have to be more than just the first bud on the next branch of the family tree if you want control of the family gazillions.

This clear-sightedness has kept the Rothschilds very handsomely housed and furnished over the two centuries of their rise from a Frankfurt ghetto to one of the richest clans in Britain. It also means that they see nosey documentary film-makers coming from a mile off.

In theory, there is much to get stuck into: their political connections; the anti-Semitic and persistent depiction of the family as Zionist racketeers; the battle between Evelyn and Jacob, both interviewed, for control of the bank in the late 1970s. What we got was a National Trust tour of the family seat, Waddesdon Manor, from Jacob and his daughter Hannah (of Nat, George Osborne's chum, there was no sign.)

Still, what a pile. The 19th-century Rothschilds, newly enriched, soon acquired aristocratic airs. The reaction was a collective sneer, yet the family's ability to assimilate has stood them in good stead, it seems. Jacob may demur at being identified as an aristo, but the family is, he admitted, establishment. It is also, Hannah reminded us, Jewish – how you wished that the programme had explored the connection between their pukka vowels and the twang of the guardians of the fading Whitechapel cemetery where, we learned, some of the family are interred.

The Thin White Duke was one of the incarnations explored last night in David Bowie – Five Years (BBC2, Saturday ****), a witty hour-and-a-half of rare and previously unseen footage from his great era. Favourite scene: from the recording of Young Americans, a twitching, snorting Bowie telling a bemused-looking Luther Vandross how to sing his (very) white man's soul music. Golden years, indeed.