Do what I did: set an amount of time that you are going to be out there, say: 25 minutes. Walk the first five minutes or so, then jog. Go until you're winded (not gassed, just winded), walk until you recover, jog again. Your goal is just to get through the 25 minutes, jogging or running as often as you can without overexerting yourself.
Over time, you will find that you can run longer and longer between walking breaks, until eventually you can run the entire interval. Then you add five minutes to your total and do it again.