I rarely take a walk with my brother, and so when I went with him on one this afternoon I remembered why: it's extremely difficult to maintain his walking pace! His "normal" is equivalent to most other peoples' "rapid, exhausting walk", and when I ask him to slow down he complains that the speed he has to adopt is "slow". I can match his pace but can only maintain it for a short amount of time, around fifteen to twenty minutes. After that, I have to rest or at a minimum reduce our speed.
It's not as though I am unfit — far from it, given that I walk almost every day, but my brother walks more frequently that I do and his walks are of greater distance. He also has a tall, light body build which gives him a natural walking/running skill; even on a hot day, provided he has some water, he can walk for miles without suffering from too much exhaustion.
Mum and Dad are slower than I am; in the rare times my brother walks with them, he says that he walks behind as to not inadvertently pull ahead by being in front. My parents also take a walk nearly every day and have improved their fitness over the past several months.
My brother has a build suited to walking and running. Am I envious? Only in that I'd like to be able to maintain his speeds for longer periods, but I'm happy with who I am. Still, when the warmer weather becomes more frequent, I doubt I'll be walking with him more often than I do currently!