Get Lucky 7K training

I have been training with a purpose lately.  I signed up around christmas for the Team Ortho Get Lucky 7k race.  I have been focusing mainly on speed training as I have always been a slow runner.  Today I did my first long slow distance run and went the full 4.5 miles or 7k oops :D.

This has me thinking if maybe I shouldn't find a longer race to run sooner than I had originally planned.  My plans were originally to do a half marathon before the end of 2012.  With these big leaps forward in my running endevours I am thinking about signing up for the Red, White and Boom race on the 4th of July.  I think I could manage to train for that race as its about 5 months out and only 3 times further than I am currently running.  My biggest concerns are that because I am such a slow runner I don't know if I could finish that long of a run in the alloted 3.5 hours.  Theoretically at the rates I am able to run at today I could finish with a half hour to spare but thats cutting it a little close for my liking.  Plus if I take too long of a walk break I could end up being picked up by the sag wagon.