If all goes to plan, Princes William and Harry will be halfway through their 1,000-mile charity motorbike ride across South Africa on Wednesday this week. Here's a suggestion for something they might do as they peel off their leathers at the end of another "gruelling" day burning up the wilderness, terrifying wildlife, and deafening the locals.

Wednesday is the day that next year's Tour de France route is announced. That's bikes minus the engines, in case William and Harry need reminding. At the same time the Tour organisers will reveal the route of the Etape – the annual event in which some 8,000 amateur cyclists are given the chance to ride a mountain stage of the greatest bike race in the world.

Unlike the Enduro Africa event in which the princes are taking part ("Fitness is not essential," says its website), the Etape requires many months of arduous training, and huge reserves of stamina and willpower on the day. It's a massive physical challenge that offers pain and exhilaration in equal measure. The sense of achievement at the end of it all is profound and richly deserved. I know this because I've done it.

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How much training have the princes done for Enduro Africa? "Absolutely none whatsoever," William says, as if he's proud of the fact. And they really expect to derive satisfaction from the experience?

So my message to the princes is this. If you want some credibility, if you want some respect, and if you want to know the real meaning of a tough day in the saddle and the rewards that go with it, check out the 2009 Etape route and pledge that you'll be there next July. I'll even sponsor you myself.

The various causes that the princes are raising money for on their motorbike jolly are perfectly laudable. But in every other respect Enduro Africa is about as crass an undertaking as any royal can ever have got involved in – far worse, in its way, than various Windsors' notorious appearance on It's A Royal Knockout many years ago, which at least showed their willingness to make complete fools of themselves in public.

William and Harry may be overnighting in what is described as "modest accommodation", but there remains something inherently distasteful about these symbols of privilege and their 80 or so fellow-bikers parading such gaudy, polluting totems of Western consumerism as their 230cc Honda CRF machines across one of the most scenic but poorest parts of the world. Such, alas, seems to be the hold over the princes' imagination of trans-Africa petrolheads Ewan McGregor and Charley Boorman. I assume it's them and not Easy Rider, anyway.

Meanwhile, real cycling is enjoying a boom, for reasons that even the princes must be aware of. And after the Beijing Olympics there's a case for arguing that it's the sport Britain is best at right now. I'll stop myself from saying that cycling is cool, but then really how cool is what the princes are up to in South Africa? Pretty naff, I'd say, even without that ridiculous war paint on their faces.

Still, it's not too late to redeem yourselves, lads. Check out the Etape, swap those seriously ugly motorbikes for some gorgeous titanium wonder that you have to power yourself, and start putting in the miles. Of course, cycling carries more than a whiff of egalitarianism. But that's not going to put off our 21st-century royals, is it?