I use a heart rate monitor when I ride and when I am indoors on a exercise bike at the gym or on a trainer at home.

Outdoors on a bike, I find it easy to maintain a high enough heart rate that I am getting a good workout, that is harder to accomplish indoors. But, using a heart rate monitor I can tell if my effort level indoors is too low so I increase it to where it would be if I was outdoors.

Indoors, a tv or something else mindless helps. Sometimes I read a novel or read the news on a tablet, but I find it hard to concentrate when my heart rate is up so I do not try to read anything that requires too much thought. My gym broadcasts tv audio on fm, I use a radio headset there. At home I have my DVR audio output plugged into one of those little transmitters designed for you to broadcast from your ipod into your car radio so I can also use a radio headset at home on my trainer.

Indoors in the winter, I usually am pedaling for 75 minutes each time, three times a week. Any less and I am not getting much work done, I find that 75 minutes is the minimum I need for a good workout. I am not sure what the physiological aspects are but I think I need to exhaust my muscle stored glucose or something like that for it to be effective for endurance training.

And about once every two weeks, instead of biking, spend time on the stairmaster, also at the heart rate I would have if I was biking. That way I am better prepared for hill climbing.