I think the first time you run a race you tend to have little idea about the pace you can run it, so I would suggest having a target in mind, aim not to go off too fast and see how you feel as you run. There's lots of HMs around, so you will have plenty of chances to do faster times. I would have thought off that 10k time that you're capable of sub-1.35 at some point. Who knows, you might go round at sub-1.30 and surprise yourself and us, but don't beat yourself up if you don't do it. I ran my first HM three years ago in 1.48, and this year I did 1hr30.28 - so fairly rapid improvement is possible. My 10k best is 42.01, so I feel I"m better at HMs than at six-milers. The good thing about HMs, in my opinion, is that they're challenging, but they don't wreck your body and your morale like a full-blown marathon (that's my experience, anyway). Enjoy.