Start with a couch to 5k program
Most start off with 30 seconds running and 1 minute walking for 20 minutes and work there way up to 10 minute runs and 1 minute walks over about 12 weeks. After that you should be able to run for 30 continuous minutes.
Here is one such program but there are lots.
http://www.nhs.uk/Livewell/c25k/Page...o-5k-plan.aspx
Or doing what you are doing on the treadmill works too, just slowly increase the running and decrease the walking. Also try to get a few outside runs because the uneven surface of pavement taxes the stabilizer muscles in your legs a lot more than a treadmill.