Having been there at the weights you're talking about, I can tell you it gets more fun as you lose weight. I'm at 172 pounds now from a peak of 321, and I'm riding at about the speeds I was riding at 30 years ago. When I was really heavy, every mile felt like a fight just to keep it rolling.
My advice (which you didn't ask for) is keep the riding fun, don't overdo it to the point you get miserable. If you have to do something more aggressive to burn calories, biking isn't your best bet. I look at the fun I'm having on my bike now as a reward for losing the weight, but I don't think I could have gotten there without lots of hours on an elliptical.