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).
There are a couple of things I would look at. Firstly, are your trainers suitable for running? If you are running in walking shoes it maybe worth investing in a pair of running trainers, as it maybe ths that this is the cause of the lower leg problem. Go to a proper running shop and they will help you choose the right pair for you.
Once that is sorted, I'd start by gradually increase the length of your run. You say you can run 5k, but no further, try reducing your speed, and maybe you could go a little further. If not, run the 5k, then run/walk another couple of kilometres.(run a lamp post then walk a lamp post). Then gradually you should be able to increase to 6k running etc.
Ideally you should be running or run/walking at least 3-4 times a week.
Did you have an event in mind, I presume, it's not about running a marathon in training?
I have to say you should see a doctor about this or at least a trainer who helps people with running injuries. I've never known anyone with this kind of problem, so I may be over reacting.
Since your walking is not too fast perhaps you should focus on walking at a brisk pace so that you walk about 40 km in approximately 6 hours. If you can do 3 hours at this pace then you should be able to run with short strides and a quick cadence without really picking up your knees (like a shuffle).
I'm not a trainer, but I think you should talk to a doctor or nurse or someone.
