If they didn't, then they would collapse before getting to the finish line.
I'm talking about regular folks, not the Olympic Athletes!
It's OK to walk a marathon, even a half-marathon.
I did my first half-marathon last September by walking and jogging.
It was such a joy to cross that finish line!
I wasn't the fastest, but I wasn't the slowest in my class either.
I've been bit by the long distance bug, and am presently training to do my first full marathon. We've got several in the area coming up, I was hoping to do it at the end of April, but I don't think I'm quite ready to do it (physically). The heart has no problem, it's the leg muscles and tendons that argue with me on occassion.
I do a more restricted Program to maintain my weight & health. I have been McDougalling for about 30 years, with a long transition, until I finally accepted this lifestyle, stayed on Program and reached my goal back in 2006.