Thanks for the replies. I think part of the problem is that I over heat, when I'm hot I struggle. I just don't sweat while I'm running, only when I've stopped and start to cool down. I feel like I can work harder in terms of my legs and by breathing. I've just done a 4.6 mile run in just under an hour with 4 min running and one min rest, I was getting 11:45 per mile.
I'm going to try differing my runs, doing some shorter to increase speed and some longer for endurance.