In reply to BusyLizzie:
I find the heat hard for running, but not aches and pains. Heat management is what restricts us from running further/faster, same as any creature, like a cheetah can only sprint for a short burst and then overheats and has to quit chasing it's dinner. Not saying I'm as quick as a cheetah, but pretty close
Obviously some of us are better or worse at coping with heat, genetics, body shape, fat, acclimatisation, all sorts of factors. Doesn't help that some go shooting off up a hill on a scorching day whilst others wallow in a pool of sweat.