Well done on going for your first HM. As others have said, given your current training (and 10K speed) you?ll be able to get round, it?s just a question of time. That being said, it is definitely worth building up to 12+ miles in training for a couple of reasons.
Firstly, you?ll pace yourself more sensibly ? training up to only shorter distances before doing a HM, means there is a real temptation to go off faster than you should for the HM distance, especially in a busy field, which means the last couple of miles may well be very painful and very slow.
Secondly, psychologically it makes a real difference knowing that you have run that distance in training, especially when it gets to the 10 mile marker or so on the day of the race (which is where I find it you begin to feel it). Knowing that you can do the distance or close to it helps you, or at least me, push through that.
On being able to track your pace, it does help, though you can get a little weirdly obsessive about it. If you can get over that (um, I think I have now!) then it?s very good as a training aid to help push yourself to perform as well as you can. On the day itself, it also helps you run to your own time rather than getting swept along by the field. Just as an aside, rather than a Garmin I?ve got the Runmeter app for iPhone 4. Only costs £2.99 and does everything the Garmin does and more as far as I can tell ? it?s fantastic and a fraction of the cost of a Garmin.
Finally, some wise words about checking the terrain of the course. Did the Nottingham HM last year and it was very hilly which I just hadn?t trained for ? some of those hills were not fun at all and made the last couple of miles much harder work than they would have been. Building in hill training (and interval training) into your regime is good in any event, but if the course is hilly, it becomes very important.
Hope this helps!