I'm not sure if you have access to other cardio equipment, but what helped me increase the distance I can run without stopping to walk and also my speed wasn't running but doing other types of cardio. When I first started running, I could only go a mile or so without stopping to walk and it took me 45 minutes to go three miles. Then I started having problems with cramps so I gave it up and started doing all my cardio at the gym (elliptical machine, stair machine, running on the treadmill, and swimming). It was probably nine months to a year before I tried running outside again, but when I did, I was shocked to discover I could run over 6 miles without stopping and that it only gook me about an hour.
Swimming especially seems to improve my running; I often see a noticeable difference in how fast I can set the treadmill during the weeks that I also include swimming in my exercise.
If you don't have access to cardio equipment, then may try running intervals. Run as fast as you can for a minute, then slow or walk for a minute, then fast as you can for a minute, etc. etc. Maybe do 10 to 15 minues of intervals a couple of times a week. Beside improving your cardio endurance, there are a few studies that show that interval training is more effective at reducing belly fat (and isn't that something all of us are trying to reduce).
Also, I always run faster with my iPod.