Hmmm; even if you do 3 minutes of running, i.e. one lap at roughly a 12 minute/mile pace, followed by 3 minutes of walking, that still amounts to 45 minutes of running over 90 minutes. Maybe I've just had to be super cautious, but I think that may be too much for where you are. Obviously, you are in the better position to judge how you feel, but I know from personal experience that an overuse injury from running can develop rather quickly.