I used to be quite a fit 50 something year old woman a few years ago, do a few miles running a week as well as a good 25-30 miles on a bike until I took I’ll with gastritis and the IBS. Now, I’m lucky if I do 7 or 8 miles on a bike and forget about any other fitness regime.I did 32 miles this afternoon and I'm shagged, never mind 256.7 km!
I have no idea how they keep going especially when it comes to the hills. I suppose young, fit, conditioned athletes is the key.