Just thought of one more thing for ya...you might want to give the Galloway method a try. It advocates a strategy of run-walk, say, 8 mins running, walk 1, etc and it can definitely help if you get plateau-ed with a running program. You end up increasing speed for the run sections & not wearing yourself out over the course of a race.

Just a thought! Good luck with the half - I'd gladly offer to pace you if I could!