Race pace can either be gauged by previous race efforts or by a time trial over the route in a training run. On race day you can expect to be faster though, so factor this in.
McMillan is useful in establishing comparable race performances. In other words, if you were to run a 1 mile max effort time trial, then McMillan would give a pretty good guide as to how you'd do in higher distance races. But bear in mind that the greater distance disparity, the less accurate it will be......but its a good guide regardless.
As for a tempo run...... a guide would be to do say 5 miles at 15 seconds per mile slower than your 10K race pace. By coincidence, 15 seconds per mile might be close to a person's half marathon race pace, but a tempo run would tend not to exceed 7 miles usually.......
