I run when I can't get out on the bike. Thirty years ago I ran a few marathons. I only do a local 3 mile run now but I sometimes walk a part of that but I do run the whole distance pretty easily. I'm big and heavy so more a plodder. I'm often confused whether it is best for fitness sake to walk fast or run. There is an old guy in the gym that power walks for an hour about four times a week. You should see the body on him! Like a thirty year old and he just seems to do stretches and a few light weights in the gym.