This workout used to be called my "Patented Treadmill Fartlek" because I invented it. I haven't done it for a few years though.
1.5 mile warm-up and then some stretching.
2 minutes at 8:30 pace, 2 minutes at 7:30 pace, 2 minutes at 6:40 pace, 4 minutes at 6:00 pace or faster. Repeat six times for 60 minutes of continuous running. Ended up with 8:83 miles and ran the last couple minutes at 5:45 pace.
Then about a mile cool down.
1.5% incline on the easier running. 1% for the faster running.
This used to be a staple of mine. I'm happy that I can still do it and that it didn't seem terribly hard.