Can I just say how awesome it is that I saw this post and immediately thought, "hmmm, I'm a runner, maybe I can respond" because two years ago, I was definitely NOT a runner and would never have identified myself as one. I considered running to be the most loathed of all forms of exercise.
OK, sorry, back on track now. I think you just have to keep at it. Try to get out there a few times a week with your run / walk routine. Keep at it until you feel comfortable and then push yourself to run a little longer and walk a little less. I think you'll be surprised how quickly you build up endurance. When I started exercising, a few minutes was all I could manage as well, but with exercising just a few times per week, it was only a few months before I could manage ten minutes of running without stopping.
I also did other exercises to improve my cardiovascular endurance, including the elliptical and stairmaster at the gym and swimming. The swimming, in particular, leap-frogged me forward in terms of cardiovascular endurance. Even now, every time I swim, I notice a marked improved in how long/fast I can run. If you have access to any of this equipment or a pool, you might consider trying some of it.
I did not do this, but I know others have had a lot of success with the couch to 5K programs that are available. Since you have a stated goal of running a marathon, this might be a good strategy for you. The longest I run is 6 miles and I only do it once a week. I run intervals on the treadmill a couple of times during the week, but I'm not a distance runner.
My Before and After Pics
"Decide what you want; decide what you're willing to exchange for it; establish your priorities, and go to work." --H.L. Hunt
"Life has hills. Set the treadmill at an incline."