I totally agree on taking days off. Ultra runners do need to take time off to recoup what super long runs takes out of your body. But I'm convinced still we were made to run. At my best I was jogging 10k+ 2-3 times a week with shorter 5k runs always maintaining a 7 minute pace, but I dont remember much of any pain associated with actually running. I honestly felt like i could do that for as long as I wanted. I was vegan at the time but also a smoker.