Pick a time to bike that can be divided by 5 (i.e. - 30, 45, 60 minutes, etc.)
Every 5 Minutes change your cadence until you reach your time limit. Try to stick with that cadence throughout the 5 minute period.
Bike computers are great at setting various paces- also, crank up the music to try and help you mentally overcome faster cadence periods!
Life on Two Wheels!
Get ready for a bumpy ride, workouts may be deceiving!