Really slow, eh? That's the only speed I know- I can go on forever, it's just speed that I'm lacking.

Not that walking is anything bad, but just don't give up on running until you know how your body will respond. Who knows- you might be able to run 3 miles a couple times a week until the day you die and never have a problem. I'm a walk-runner most of the time (run a few minutes, walk a half a minute). Nothing wrong with that either. Whatever gets me down the road on any given day.