My first race was September 2003 after a few months of running (4m race - done in 27:51). From there I slowly moved up into distance in 04 and got bitten by the marathon bug and ran my first marathon down the cape in 04 (3:28). From there I caught the Boston bug and trained all of 05 for Baystate and got my BQ. I personally think you plateau in running/racing like you do in losing weight. I plateaued personally after my first Boston and didn't get over the next hurdle until I increased my base mileage. In 07 I averaged 62mpw and I think that definitely improved my racing - that along with a steep decline in racing.
I would be curious to know if your mileage has also plateaued because over the last couple of years I attribute increased mileage to my performance. Also, I just turned 33 and know I can get faster - I'm sure you can to, still being a young runner with fresh legs.
Ahh, the easiest way to get hurt
. If you try to train for speed and distance at the same time, you generally improve neither. If you think about it, if you focus on a solid running base and throw in a weekly track workout you will naturally get faster at the shorter stuff and this will help your distance performance. Mileage for distance - track for speed. The way I look at it - I try to build my speed first at the 5k level (say 5:30 pace) and then focus that pace on 10ks and 10M, etc,etc.
Sometimes you need to 'shock' your body into running a faster pace by using track as well. If you focus too much on slower training runs sometimes you are less likely to get faster.