I am looking for some fun and 20-30 bike workouts that I can do on Tuesday/Thursday for my "off" day from running. Any ideas?
With bikes, rowers and some other cardio machines - i like using it for interval training, starting with a 30/30 work/rest ratio.
The good thing about the machines is you can monitor the intensity. So each week i will up the work time slightly and lower the rest time, whilst maintaing the intensity of last week, which you can measure accurately with the machines.
When you get bored of working in that direction for a while. Go back to the 30/30 or whatever you started with and see how much you can increase the intensity. That's how i like using the cardio machines anyway.