For the gym cardio equipment, I just mix things up from day-to-day or even on the same day. I'm pretty happy on a treadmill for an hour, but I do a mix of running and fast walking at a 10-15% incline. The incline has less impact on my joints, but gets my heart rate up into a good steady range. Other days, I might do 20min treadmill, 20min rower, 20min stepper. Keeps me from getting so bored.

Also, don't worry what ranges are shown on the gym equipment. I've never understood how/where they get their numbers. I think its done to make users feel happy about themselves. Get yourself a heart rate monitor and learn what your own ranges are. Some gym equipment will pick up your chest-strap and display it which is nice.

I think that one misconception about the lower heart rate = more fat burned is that at a lower rate you are burning a higher percentage of fat. However, if you burn 100 kcal per X minutes at a low heart rate you might burn 45% fat = 45 fat calories. Push harder and your fat burn percentage can drop to 35%. But if you burn 200 kcal over the same time period you've burned 70 fat calories. So unless a doctor tells you differently, I'd say to work as hard as you can keep a steady pace (see LSD explanation above).

One last comment about hard intervals....you probably don't want to be doing these every day as you need to give the muscles more time to recover and rebuild. But they are indispensable to getting in shape, getting faster, raising your athletic capacity.

(PS: Most of what I've written has been learned in the past six months while dropping 30 lbs at a consistent 5 lbs a month while increasing my riding speed & distance and strength through weights. I can't say it's 100% correct but it's worked for me.)