You're not used to moving your legs that quickly. Try counting 1,2,3 every second (not while running - while reading this). That's how fast your footfalls should be. Generally we're a lot slower on the fells, so when we get to a runnable bit we struggle to up-tempo.
That's what's worked for me anyway. I do 6 lots of ~600 metres every few weeks. This really seemed to help my leg speed.