I wouldn't call 90 minutes a long ride, but leaving that aside...

It's complicated. I can promise you that you cannot put out and sustain enough power to burn as many calories in a half-hour as you are currently burning on your 90-minute ride. You'd have to be anaerobic the whole time and that can't be done. However, the 30-minute workout you describe, despite burning fewer calories, will give you some benefits that your longer ride doesn't. Interval training will tend to push up your threshold and, over time, increase your cruising speed. The answer is you need to mix up your training and do LSD - long steady distance - most of the time, with periods riding close to your threshold (the amount of effort you can sustain for an hour) and some interval sessions. Personally I wouldn't recommend doing intervals more than a couple of times a week, you need to recover effectively.