I've built up my walking over the last 4 months and done a little jogging. I would walk for about 4.5Km in total, after 1Km I'd jog for about 100m then walk another Km and repeat it. I'd use an objects as markers. Basically eased myself into it. I think my general fitness level stopped me doing too much, I was so out of breath, but I'm going to have another go at it soon. Why not jog the home stretch of your walk to start with.