<please excuse if this sounds abrubt and rude, I'm trying to be helpful
>
There's a couple of potential dangers for Chris I think ;-
You just get better at plodding, so you can run at 5mph for 30 mins, but don't get above that, and you are not utilising all your running muscles (hammies, glutes, calves, quads) and just relying on eg your hammies to pull you along so you get overdeveloped in that area.
So a suggestion - try varying it a bit more, walk a bit, do some jumping jacks, run a bit, hop on each leg, couple of squat jumps, bit of skipping, bit more walking, bit more jogging.
Then when the running bits get longer do some bounding up hills - just extending steps up a gentle hill to get the glutes firing.
Down side is that you'll feel a bit of a prat
, but I think your running will pick up quickly.