That's a nice explanation Nor Cal and I think it answers the original question.
But speaking of not doing tempos too fast, is it better to be slow? If my tempo pace is 6:00 is 6:10 better than 5:50. Does it change things at all if I'm unfit at the moment. My tempo pace should be 5:30 or quicker, but I think it's closer to 6:00, but don't really know. In this case is it better to go faster to get used to 5:30 again, or even more important not to go too hard if I'm unfit?
We called them combo-workouts back in the day. Something like a 9 miler with 3 miles easy, 3miles at tempo pace (10mile/HM pace, no faster) + 2 X 800m hard (like 8km-5km pace) + 400m, 300m, 200, at 3000m pace or slightly faster as the reps got shorter. The reps were done with about a 2min rest. Then a cool down of 1-2miles. Some days we'd end up hammering the last few reps at closer to 800m pace if we felt good and the coach gave us hell. The faster we ran the last few reps the worse we felt the next day.
To be honest I am one to fault on the fast side...as I think most runners would, should and do. But what is best for your body physiologically? Probably to fault on the slow side, and leave a little bit extra for the race. But there is no fun in that!
If your "tempo" pace is 6:00...5:30 is 10k pace? Different workout!
PS. I left out all the big science words on purpose of my explanation, but can add in some of the science if you like. As someone with a masers in exercise physiology I get annoyed by some of the posts on here that vomit their college text book and try to sound smarter than they are.