I returned to Runner's World's Training Calculator to see how my 15K race time of 1:15:12 predicted a marathon - it came up with 3:44:55, which is a little better than the 3:49:46 predicted by last year's 24:00 5K PR. This 15K time also predicts a 23:28 5K - which would be pretty sweet. My long-term 5K goal is to beat 20:00, which I think is possible within a couple of years. The Training Calculator needs a 6:00 mile to predict a 19:57 5K - and it would be awesome to race at that level of fitness, a VO2max of 48-50 (3:11 marathon, anyone?).
If I can keep up with the training plan, peak at 40-45 miles/week before the marathon and work out these nagging leg/foot issues, it looks like these race times are well within reach. It's probably time to tweak the training paces as well so I can stay on track and get a little faster.