Yesterday my plan was to run my usual 2 x 4 x 200 meter hill workout.
When I got there I decided to extend my hills about 50 meters and run them harder than usual.
Did 3 x ~250 very fast with walk back recovery. (First two felt extremely quick, third one was decent but I was dead afterwards)
Rested 2 minutes then ran 1 x 250 following by a 150 (like a split run)
It nearly killed me but it felt GREAT.
I felt really strong after I warmed up so I decided to step it up a bit. HUGE lactic acid workout and the split run was awesome! You can concentrate on running fast while fatiqued and still maintain good relaxation.
If you are looking for a change try something similar. 
Cheers,
Chris