The concept of an unfair advantage is deeply problematic.
Why don't we ban athletes who eat healthily, or train hard? Or who were born with a great physique? If one of the runners in a race was a smoker, why not ban all the non-smokers from racing against him? How about athletes with money to burn on fancy therapies and sport scientists to monitor their every move? There's no such thing as a level playing field. We make all these artificial distinctions (like not having women compete against men) to try and ensure there is, but we're kidding ourselves.
It's hard to see how we might draw a clear line between any of these things and e.g. letting one of the 'runners' use a bike. There may be a case for saying as long as you're actually *running*, anything goes. The alternative would just be *anything* goes - here's the start, here's the finish, first to cross the line wins - like Wacky Races. Before you laugh, think about the competetive advantage being smart gives you in many sporting situations. Is that unfair? If we want the purest form of competition, then giving people's ingenuity and dedication free rein seems to makes sense. Survival of the fittest is a question of environmental adaptiveness, after all.
And it wouldn't half be fun to watch.