Indeed thanks to Coach for all the great advice. I have a question to add to the mix too.
I have been running for 5 years, never been a speedster, still 35 lbs overweight. Last year I had an Achilles injury (not ruptured, but very sore). I took some time off, did a lot of walking on the TM over the winter. Back to running outside in March. I am currently on week 2 of the Higdon Novice 2 plan for an October marathon.
In the last few weeks, my pace has improve dramatically. My half marathon PR is a 10:16mm pace, and I am currently averaging around 9:45mm on runs 6 miles and under. Last weeks long run of 8 was 10:43, which included me tripping and falling and a very hot and humid hilly course.
I am not sure where this new found speed is coming from during my shorter runs. I am not actively trying to go faster, I just let my legs go how they want to. On todays 5 miler, mile 5 was 9:18

.
Any thoughts? Did the rest over the winter help? I am by no means complaining about the increased pace, but I just don't know where its coming from. I have not changed anything else, and I am running better in week 2 of training than I was when I trained for the DL Half in '10 (which is my PR). I weigh about the same as I did then. And no my legs have not gotten any longer.
Again, not complaining, just trying to understand why and if I can/should expect more improvement. Not that its likely, but the October marathon I am running is one of the top Boston qualifiers. I would still have to improve substantially, but losing the weight and training well, I might get close.