Fall from grace only meaning that the runner was once at a very high level, then due to some circumstances became a very weak runner. I probably should have made that clear in the post, so my bad. Looks like Alan Webb is the victor if we're going by majority vote.
You don't understand the expression "fall from grace". It suggests a loss of standing or respect and in particular a show of poor character. You should simply have said, "who showed the biggest decline in performance".
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.
I would argue Bekele had the greatest fall from grace only to rise back up. Went from wrs in 10000m and 5000m in the early 2000s, to a few DNFs and DNSs in the marathon in late 2010s, to now in the early 2020s we are talking about him as one of the top marathon contenders. His career has been legendary.
Erick Wainaina, from 2-time Olympic marathon medalist to arrested this year for molesting a high school girl on the train in Tokyo and punching railway staff who tried to stop him.
Fall from grace implies a very high level of public knowledge and support. I've followed T & F for 30+ years, and I'm not sure any distance runner from the US was socilally relevant enough for this to occur. I know Webb was on Letterman, but it's a flash in the pan. Something akin to the person that can juggle the most balls at once.