Marathon on a bike

Yeah, but you're looking at the differnence between running fitness and cycling fitness. I'm figuring a five hour sustained 5mph run is the same as a five hour sustained 16mph ride. I guess it depends if you treat a marathon as a race of a challenge. If it's a race at the top end of athletic ability then you'd be looking at around 60 miles in just over two hours.

I am admittedly crap at running but even after 20 minutes at 5mph I am feeling it, whereas 16mph on a bike is still pretty much coasting. Would need to be faster than 16mph to ensure effort is always required as you cannot run without effort otherwise you stop!