Thanks for the replies. I'm running about 3-4 times a week. In the past I use to just go out and run 40-50 mins, but now I'm doing shorter runs (<5 miles) with pace running sections of approx 5 mins. In the past I've run a half marathon in 1hr 40 mins, a 10k in 44:40 and a 5 k in 21:10.

I'll look to change my plan based on all your comments, particullary team-lewis and LazyJones.