Are you getting bored with your steady state cardio exercise? Is 40 minutes on the bike numbing your bottom? Try this style of cardio to up your aerobic capacity.
Choose any machine and start on an easy level.
After 3 to 4 minutes increase the level by 2. One minute later increase by 2 again and then every 30 seconds increase by 1 level until you reach a maximum level you can cope with. Work at that level for 30 seconds before you drop to the level where you started. Spend a few minutes on an easy level recovering before repeating.
You can then repeat this as many times as your want or can cope with.
An example would look like this:
Bike at Level 3
Reynolds @ Rainham