I'm amazed by how much my running speed has increased since the half marathon. In January my average pace per mile was 15:46. That included some very slow long runs but even my fastest three mile run was at a 13:24 pace. In February my average pace was 14:11, in March so far 13:52. Yesterday I ran 6.2 miles at an average pace of 12:48. Last year my goal was to run a 5K at a pace under 12 minutes a mile--this year it looks like I may be able to do a 10K at that pace.
I think the half marathon had two effects. One is that as I pushed myself I learned to run less tensely. The other is that I discovered my body is a lot less fragile than I thought it was--I can push myself more. I had some reason to worry about running, both because of a history of knee injuries (particularly a kneecap that used to dislocate) and because of the hip pain that I struggled with when I first took up running. But I've become stronger than I realized.