I have very little experience with running, actually last X year I was running only to catch a bus. But I walk a lot, I make 40 km without a problem.
However, when I've tried to run, I've finished after 5 km. Without a breath, with a pain in my legs (lower parts).
How should I train running under that circumstances? Making a distance is not a problem for me. The problem is trying to make a distance faster as usually (typically, I make 40 km in 8-10 hours walk).