With a plan. It gives you something to aim for, but you have to be easy on yourself if you happen to miss stuff. Your plan for a half should involve a couple of longer runs, which will be the long one (usually on weekends known as the LSR) and a slightly less long one midweek to back it up.
If you've run a more recent race than above, plug your times into something like the McMillan calculator to give you some training paces. This will give a good idea of what paces you could be running your training runs at.
It is important to go faster sometimes. As March approaches you should be planning to get a run in each week that has an increasing few miles at your race pace (as according to the calculator or by how you feel). Through Dec and Jan you might throw in a Fartlek run once per week as this is light on difficulty but entertaining, and you can gradually progress this in towards race pace. Strengthening is also a possibility if you have time, so finding some gentle/not so gentle hills on your routes will get your legs a little stronger.
A lot depends on how much time you have available to train - whether you're getting closer to those 4 runs per week yet!?