Webb. Top of the world to suddenly barely able to break 4. Even in that era, that wasn't enough to continue as a pro. Webb attempted to come back to track for several years without real success.
He then tried the triathlon for a couple years but couldn't make it to the upper echelons of that sport.
His comeback attempt was like 6 years long.
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Steve Magness - 401 high school miler. Never pr'd again after that in the mile or if he did he didn't even take another second off. He ran in college and post collegiately for a couple years. His pr's were few and far between after hs. He brought his 800 about in line with the mile and dropped a little time in the 5000. I don't think there has ever been a runner so good who didn't get a big pr in college.
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Bekele - the best distance runner in the world by far for years. He could not duplicate that success in the marathon. He has had some good marathons here and there and after years of sub par results clawed himself back for one more attempt at the olympics. The deprioritization of world xc really hurt him. When world xc was big, he was the most dominant xc runner in the world.