Stutyr's adice is good, as is David Falconer's. But if you're secretly wanting your 'charity-run-goal' to be sooner rather than later, I would add that you have plenty of time to get running a decent 10K in May, if you put the amount of training you describe. In fact it's more than enough.
3 midweek runs and a longer one at the weekend, coupled with your resistance training will have you fit as a butcher's dog by May! Unless, of course, you do too much too fast, and your injury returns... so just be sensible.
Definitely do a May 10K... and then an October half marathon. Choose which one you want to be your main fundraising event. Well done, and good luck.
EDIT.. I just saw stutyr's 2nd post... more good advice! But as a teacher, even a newly qualified one, I'd hope you can find plenty of summer time to get some good training in!
Edited: 04/02/2013 at 10:28