Intervals are a great way to start. If you are using a treadmill, most have a interval program setting - this makes it so easy! If you are "street walking," time yourself walking for three minutes, then slow-paced running for a minute, then walking for three, then running for one, etc. Try this schedule for a week or so, then boost up the running time incrementally until you are running more than walking. Take it SLOW! Play with running speeds like sprinting for a minute, slow-paced running for two, walking for two, etc. I just read interval running can actually burn more calories in a shorter time! Tell yourself what you are going to do for how long before you do it - this helps me, too.
I was never a runner, but by taking it slow I can now jog (my pace isn't fast enough to be considered running :) ) for an hour stretch... It feels so good when I finish the workout. I accomplished something I didn't think I could do!
Hope this helps.