So many keeps I've read over the years indicate using the mill for X amount of time at a nice, steady pace. While that's not a bad idea, I think the idea of using actual sprint work in the keep is beneficial. I was having this type of discussion the other day with some friends, and that lead me to actually do some research on the subject. Here are some studies for anyone that feels compelled to learn:
http://www.ncbi.nlm.nih.gov/pubmed/21849912
http://jp.physoc.org/content/575/3/901.full
Granted, it is geared toward people, but it can be applied to dogs without a doubt. The second one is a little wordy, so just be warned.