there are several FREE running apps with programs to help you
Fartlek (all out sprint) runs helped me. I never had much problem with the endurance (granted I only run 5 or 6 miles at a time), but never could get the speed. I alternate long runs and Fartlek runs.
And if I planned a long run but I'm just not feeling it, I change my plan and do a Fartlek run.
I'll always be a slow runner, but my time has greatly improved since I started doing this.