Timed Intervals 

2x20 minutes
1 min on //1 min off
24//26//28//30//32 - 2 reps at each rate
Rest for 6 minutes and repeat

Lower the drag factor 7-10 ticks below your normal spot. Keep the legs firing, be quick at the front end and don't worry so much about the split as the rate. Put in a good warmup to get used to the higher rates and get after it. Don't compromise your technique as the rate increases - keep the order of operations.