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.
I've been running since the end of April, starting with the C25K app and moving onto the half marathon trainer. I'm doing well with it but can't seem to increase my speed on my intervals. I always seem to have an overall time of 12 minutes- 12 minutes 30. Only once have I had less than 12 minutes. Am I just not pushing hard enough to get a faster time? I think my fear is pushing too hard to get a better pace and then running out of energy to finish the intervals.