In reply to mattrm:
> So for 1:30, if I was doing 800m in 3:00 then, I'd be able to run around 1:30.
Yes, but ...
> So if I'm a bit slower than that, say 4:00, I've got 6 months to sort that out.
There's a very big difference between 800s in 3 and 4 minutes. If you want to do a decent time - and there's no reason why you shouldn't - you're going to have to learn pacing over the short distances - Iain's 800s - and over longer distances.
Laps on a track are good for learning pacing for the shorter distances, but for the longer distances, you'll either need a GPS watch or you need a known route where you know the mile marks. Work out a route and then work out timings for different speeds. It'll be a gradual process of increasing speed, but you've got plenty of time.
For a 1.30 half, you need to be averaging a bit less than 7 minute miles. You might find this hard to achieve when you're training over the distance, but race psyche will get you another few minutes. Last half I did I was training at about a 1.35 pace, was expecting to do about 1.32, and actually did 1.29.