The question of what is meant by "rest" days often comes up. I just read this: about 3/4 through he describes what athletes do. The answer for us seems to be "not run, but not nothing". Getting the blood circulating using muscles and movements other than the ones which are recovering from injury. So, walk, swim, dance, cycle. Not to hard.
The article is a few years old, and it's main point is correcting thelesson many of us may have learned about why you hurt so much AFTER new and harder efforts. So, this is a good article for newbies of a certain age, carrying out of date information in their head.