You don't need to ride for more than 20 minutes to generate results. The time-proven, aerobic school of thought has shown that hitting one's Target Heart Zone for up to 45 minutes is ideal for utilizing fatty acids as a fuel source.
I.E. you're hopefully burning more fat than glycogen or, worse yet, protein.
But you don't have time for that, and even if you did, steady-state aerobic exercise will almost undoubtedly lead to a training plateau.
Why? Because the body adapts, and that's EXACTLY why working out WORKS. But the problem is that your body adapts, but we keep doing the same exercises that the body already adapted to.
This lead to the discovery of High-Intensity Interval Training. Unlike steady-state aerobic workouts, HIIT stresses our anaerobic threshold.
Here's a quote from another trainer I happen to agree with:
One of the physiological functions/outcomes of interval training is an elevated Excess Post Exercise Oxygen Consumption rate (EPOC). With this, though you burn more calories doing SS cardio over HIIT, HIIT elicits extra energy burn for up to 24 hours! This is huge when you are trying to lose fat. Remember, altering your body composition really comes down to energy (calorie) balance. Using HIIT will help you burn more energy once all is said and done.
Now, there is something that you need to know about HIIT, though, and that is the degree of difficulty in performing it. There is a difference between HIIT and the more-simple IT. The difference is the "High Intensity."
Done properly, HIIT will make you feel like total garbage, you'll lose finer motor functions, and you'll probably want to vomit. It's also not something that I would recommend to a deconditioned person to go out and attempt... you have to be in reasonably good shape to perform at a high-intensity level. Since the goal is reach max heart rate, you don't want to give yourself a heart attack. Be smart and realistic about your fitness level and build up slowly and safely.
Another thing to keep in mind is that HIIT is utilizing the same energy-pathway for fuel as weight-training, and it can also lead to the same overtraining. Personally, I don't do it any more than 3 times per week.
But the benefits of HIIT are proven, substantial, and they're derived in only 12-20 minutes. Better yet, the EPOC will turn your body into an engine that feeds off adipose fat stores long after you're done working out.
Just make sure you're consuming enough calories and protein, otherwise you'll catabolize your own muscle for fuel.
p.s. Sorry I'm a year late, but I just joined this forum last month :)