Hal Higdon's MARATHON TRAINING GUIDE
"Tip of the Week: What you do in any one workout doesn't matter. The most important point of any training program is the totality of that program, and the results it brings. A flash speed workout with quick splits may look good in your training diary, but it could bring you to the edge of overtraining. The same with running the long runs too hard. Your time in the final 20-miler won't count three weeks later. Your success will be measured by, 1) finishing the marathon, if you're a beginner, or 2) finishing it in a time that reflects your current capabilities, if you're an experienced runner. What you did while getting there doesn't count."