Have you tried using a Heart Rate Monitor? That makes all the difference for me. You can even see a guesstimate of calories burned.

The "rule" that I learned from running is that you would burn the same amount of calories going for a half hour run as an hour walk.

With more resistance (heavier bike, more hills, harder gears) you will build muscle and adding muscle increases your calorie burning capacity because muscles need fuel to work.

So in the end you could ride all day somewhere flat or you could go for a tough hilly ride at a fast pace and get a better workout because you will burn calories for a while afterwards.

In the end, it is best to do both long slower rides and shorter tough rides because your body will probably respond best to changing things up.