Run/walk at the beginning. So say Run for 6 minutes, walk for 4, run for 6, walk for 4, run for 6 walk for 4 and warm down. Do that 3 times a week. In the other 3 days of the week (always take a day off), just walk for 30 minutes or so. Gradually change the proportion; more running less walking. Then stop walking altogether. Do it gently, never increasing anything by more than 10% or so in a week. In 10 weeks you should easily be able to run for 40 or so minutes at 8 minute mile pace.