A couple of years ago I was coming off 5k and half performances of 16:14 and 1:16 (while specifically doing marathon training), but bombed the full with a 3:01. Was averaging 6:20 pace no problem then just fell apart at mile 20. I jumped into another marathon a couple months later and ran intentionally slow (6:45 pace) just so I could at least break 3 hours. Finished fairly comfortably in 2:56, but again that's pretty bad for a 1:16 half guy.
After that, took about 2 years off due to knee injuries/surgeries, and slowly got back into ok shape after a year running again. Decided to try the marathon again. Training indicated that 2:55 should have been no problem, my tuneup race was a 27:59 8k a few weeks before the marathon. So I cruised through 6:40 pace for 20 miles and then completely fell apart again, stumbling home in a 3:12.
Why can I not make it more than 20 miles at what should easily be my marathon pace?