As much as I hate to admit it, I think tempo running is best done on a treadmill. I generally hate the treadmill because I just get so damn bored. I also continuously look at the clock, which seems to slooooowly tick by. Anyway, back to the topic...It's just way easier to go at a specific pace for a certain time. My HRM watch will tell me all that I need to know, but I've got to try to maintain that speed on my own plus I have to always be looking at my watch, which will slow me down. I think a treadmill, for tempo runs, will be much more efficient in progressing my training. Plus it's harder to cheat.
So last night, I did two different variations of the tempo run. First (after my warm up, of course) I upped my pace to 8:47 from 9:50. I ran at that pace for 3 minutes and then bumped the speed back down for about a minute. I did this a few times and then went on to phase 2 of my tempo workout.
We have an indoor track at our gym. 10 times around equals one mile. So I alternated running at a pretty quick pace and running an almost full out sprint, lap after lap for a mile.
I think that, generally, you are supposed to maintain the tempo pace for a longer period of time, but sometimes a good sprint just makes you feel good. And that's just what it did.