My second one felt so much better than the first. I think perhaps because it's not quite such the great unknown in your mind. You know you can do it, and have done it, and what it's going to feel like. I can see how that would also work against you, but I wasn't so nervous second time around as a result of knowing what to expect, and I ran a much more even pace (rather than pushing it too much at the start), which resulted in a quicker time.
I ran a very even pace first time around, so that's another reason not to do it again.