You make it sound like you started from scratch and are up to 8 mile runs in 5 weeks. If that is the case, you may have started out a bit fast.
I certainly havent run at all since the summer and have never run more than 3 miles in one go. I am however following a speicific training program devised by the university of Iowa.
It is created to get non runners to finnish a marathon in 16 weeks. The training is meant to minimise the risk of injuries by keeping the mileage to a minimum.
All the best
Gareth
