I read somewhere in an interview that at age 50 Coe ran a 54 sec 400m and said he felt like throwing up.
Who knows if it's true but it sounds plausible, and since this thread is all speculation, that's good enough to proceed.
A 54 sec 400m is roughly worth a 1:57 or 1:58 800m, which in turn equates to 1:41 high or 1:42 in the age graded tables. (I'm not going to pull a ventolin and claim spurious precision.)
Surprise, surprise! That's about Coe's pr. So we're in the ballpark.
Kipketer is much younger, about 37. If he's maintained minimal fitness, e.g. jogging 20 miles per week, my guess is that he could run around 1:52. Throw in 6 weeks of 200m and 600m repeats, and maybe below 1:50?
Another way to look at it: remember how hard it was for a master to break 4:00 for the mile? For example, Walker and Moorcroft both tried but broke down before Coghlan broke through.
The rule of thumb is that you need 1:50 speed to break 4:00. That's hard at any age let alone 37 when you're on the cusp of really losing speed. Granted, Kipketer wasn't a 1500 man, but it does indicate the difficulty of approaching 1:50 at his age. Normally, I'd say "no way". But Kipketer is special.