Run Training

I'd agree with Scout.  3X "regular run" in the 4-6 mile range, 1X mid run in the 8-10 range, and 1 long run in the 12-14 range.

This would put your weekly total between 32 and 42 miles.  That alone will help you improve your time in your next half.  Once you get a solid base you can start playing around with some speed work, hill work etc.

I'd also suggest that you look at how you map out your weeks so as to not burn out... if you're able to do two or three weeks at the higher total make sure you use the next week as recover, and do the lower total or even less (say take an extra day off and only run 2X, 1X, 1X).  Recovery is just as (if not more) important as the hard training.

Agreed on all this. 

Given where you are right now, you'll probably make great gains simply by putting in some consistent mileage. If you can build to the point where you're doing what these guys are recommending, you should make good progress in your next HM. Elaborating a little on Sous' suggestion, if you plan to do some kind of race or other test about once a month, then you could use that to create the alternation between harder and easier weeks, with an easier week leading up to each race/test.