One workout I liked was 4x 5K repeats, starting at marathon pace with a recovery lap between, and the last 5K at half marathon pace. Didn't make me any faster, but it felt like a good workout.
The 20 miler before tapering was kind of a waste of time, once I already knew I could run that far. I could've done double digit miles every day for a month, and my calves would still cramp at mile 23, and I'd be back to 6 miles 2 days after the race. What finally got me under 4 hours was KT tape on the calves, and running double digit miles at sub-marathon pace every other day for the 2 months leading up to the race. Perhaps unorthodox, but whatever.
I also liked running half marathons on back to back days. It gave a decent picture of what 26.2 would feel like.
One other test. Do a 10K time trial early in your training. Do another one a few months later. If you aren't faster, your marathon probably isn't faster either. Tons of runners hit PR's in other distances while training for the marathon. I did this in the 5K and 10K, which means that now I'm more into track, my 5K PR is likely set. Oh well, it was a pedestrian 21:51 anyway.
94chem,
That, sir, was the greatest post in the history of TexAgs. I salute you. -- Dough