How much more can I improve?

I have only recently gone back to running and I can currently run 8 miles in 1 hour whilst wearing alot of warm clothing. I'm thinking of entering a half marathon but only really want to do it if I can get up to 9 miles in 1 hour. From people's experiance, how much do people actually improve when running in races as apose to going out for a normal run. I'm an 18 year old male.

Thanks for reading.

Hi Smair, most people will find that they can maintain a faster pace in a race than they can do in training, due to the adrenaline and people around them.  Have a go at a couple of shorter distances (e.g. 5k or 10k) and if see if you can maintain or beat that pace in these events.

Everyone is different so the variance between training and race paces does depend on the individual and their level of training.

probably not this good

http://www.runnersworld.co.uk/forum/general-running/becoming-world-class/226524.html

But if you're prepared to patiently build up, and put a stack of work in, you can be pretty good.

Aren't you the chap who wanted to be a boxer as well though?

Thanks for the answers and no i want to be a professional footballer or runner.