Runner's Forum: Running Program

I'm looking for some advice. I'm a 40 yr old male, 5'10 and 220 lbs. I have been "jogging" off and on since high school never doing more than 3 miles. In the past, injury or "overdoing it" on a run made me stop. This year, in February, after I turned 40 I decided to commit.  I was not sure what I was committing to but I was committed.  When I started running I was 240 lbs.  I have gradually over the last 6 months lost 20 lbs and am up to 8 miles on a typical run.  The one difference now as opposed to the past is that instead of running 3x a week @ 3 miles I am running 8+ miles on the weekend and 5+ miles once during the week.  I find that by only running twice instead of 3 xs, my body is holding up better.  My question is that if I want to prepare for a half marathon, can I continue at this pace or do I have to modify my regiment?- thanks for the help- CP