I know Steve's advice about not starting too fast is the sensible option. But I also thought I'd pass on my personal experience. I have been a very late starter to running, but having volunteered to take part with folk from work in the Edinburgh 10K in 2006 I took my training seriously and was well prepared. So my prep gave me confidence, anyway at the start I was beside a guy a fair bit younger than me, and a girl closer to my age but with a lot of running experience. We initally ran togehter, and I could feel the pace was a bit faster than my usual training had been, but I felt OK. Then as we passed 1k, 2k the girl commented on how fast we were going. I started to think I'm going to die soon, this is way above my norm. However although I did slow a bit (anybody who knows the Edinburgh route will realise everyone slows once you hit the hill in the park !), but nonetheless I was able to keep going at a reasonable pace. To my delight I finished in under 49 mins when I had put down 55 as my estimated time. I'll admit before the race my intentions were to start conservatively, but the mixture of adrenaline and running alongside my friends knocked that on the head, although with a good outcome for me.