go with about 3 miles and try to keep it under 45 mins.. that is about the pace you want.. and to help with intensity jog for 2-4 mins, run for 1-3 mins, and sprint for 30-60 seconds, and repeat. this is, in a nut shell, what HIIT training is. I think there is a sticky on the training forum about this and if not google it. start out longer on the jog and shorter on the run/sprint to start with. then build up your endurance by adding more to running and sprinting.. but try not to go over the time for running/sprinting it could hurt you by eating muscle away.. remember long distant runs vs. sprinters