Variety keeps me motivated.You can run intervals on the track, from 200 meters to 1600 meters, with limited rest between each repeat.You can even do a staggered interval session at the track.Something like this:
800 meters x 1 rep; rest 90 seconds
600 meters x 2 reps; rest 75 seconds after each rep
400 meters x 3 reps; rest 60 seconds after each rep
200 meters x 4 reps; rest 45 seconds after each rep
You can do stair sprints, hill sprints, fartlek runs, etc.Run in local 5k or 10k runs.
Get some buddies together and do an Indian run; run in single file formation while the last person in line sprints to the front.Once he gets there the last guy in line sprints to the front.Keep repeating this until the run is finished.Basically, someone is always doing a sprint and the run is a combination of distance work with short sprints interspersed.The length of the sprint is dictated by how far apart each man is spaced and how many people are involved in the run.
Lots of options to add variety.