Take this with a grain of salt because I'm a novice compared to most here, but I'll play devil's advocate and say bite the bullet. You can always back off, no? Of the 3 marathons I ran in the last 2 years, the pace for ## 1 & 2 found me, not the other way around. For the 3d, though, I had a time goal (BQ) and set off exactly at that pace figuring I could ease off if necessary. I wanted to build a 60-second cushion if possible. Well, I found myself at mile 20 exactly on pace but tiring. But I knew from my threshold runs that I could get to the finish line in time and made it by 70 seconds. That was all I had. If I'd started off 10 seconds per mile slower, I don't think I could have made that time up. It's easy today to say "I'll just pick it up with 6 to go" but when you get to that point it's a different game isn't it?
I know blowing up sucks, but you might just have had a crappy day.
(BTW, my 10k PR was a little over 42 and my marathon was 3:20 and my weekly mileage was generally in the mid 40s with a max @ 51 or 52)