Like with any form of training, you just have to decide what you're doing, whether HR or pace and just stick to it, whether it feels hard to speed up or slow down.
Not exactly rocket science.
Actually though, that's quite an interesting point (hence the edit), if you're struggling to maintain the appropriate pace at the moment, how will you manage if you're picked and why will that be different?
Edited: 10/12/2011 at 10:01