I experienced this so badly in the early 2000's that it actually showed up under x-ray. This is a sad, but true story:
My 5k PR was 17:58, from high school. I ran off and on after HS, and was reading letsrun the entire time. As a very misinformed 19 year old who could probably run a 5k in 19 minutes at the time, I worked up to 50mpw, then 60, 70, 80, 100, 120mpw almost all slow. Sometimes I would hammer.
Sometimes I would run 20 miles on Fri, Sat, and Sun (each). I did this for another year and decided to race on a Saturday morning when the dew point here in the south (in July) was 77. I ran a 17:40 5k after running 15 miles the night before the race. Not too bad, I thought.
Fast forward 6 weeks and I had just completed a 13 mile run on some very hilly terrain, off-road, in 81 minutes as my second run of the day. This was still August, and it was still hot and extremely humid.
I felt I was rounding into great form, but after that 13 miler, something just wasn't quite right. My groin felt terrible and I could hardly lift my leg to go up steps. I had registered for a 5k that was 10 days out. I hobbled around during this time, running as much as I could. I remember starting the 5k race ok, but I kind of blanked out and I remember limping (as in dragging my leg behind me) and I ran 17:55 - still hot, still nasty outside. I was done - I could barely walk. I couldn't stand in the shower on one leg. That is when I went to the doctor and the x-ray confirmed my injury.
I was in the pool immediately (as you could imagine any obsessive-compulsive person would) but I seemed to favor the stationary bike a bit more. I spent Sept, Oct, and two weeks in November riding the bike. By mid October, I started walking about 1-2 miles each day on top of biking. I started running in mid November and (I sigh.......youth....stupidity....) decided it would be a great idea to sign up for a local half marathon. I ran 1:25 off of two weeks of running and by the grace of God, did not get hurt. I was very lucky.
I think I put it behind me. It has been 13 years. I don't know if I can say the same about my endocrine and muscular-skeleton systems. I never ran another week of mileage that was as high as those couple of years. I don't know what I was capable of running in perfect weather with a healthy body. It would have been a heck of a lot faster than my 19 minute starting point and a heck of a lot faster than the 17:55 I ran on one leg.
I blame LRC, and of course, my youth and ambition. Luckily I learned a lot from the experience. I did not learn enough to stop reading LRC, though.