You will get out of it what you put into it.
Feeling like you will die in the first 15mins will go away with time, but you have to keep trying.
That's where you benefit from an instructor pushing you to keep going (as opposed to just sitting on a bike plugged into your headphones).
( CAUTION: Super rough mathematics ahead )
I think 600 calories is a conservative estimation, if you're working at about 80% of your max heartrate* ((220)-age X 80%) you will burn calories.
(*I'm guessing, it's been a long time so please correct me if you know)
Calorie wise, it's estimated that a kilogram of body fat is about 7,000 calories, and you shouldn't lose more than a kilogram per week to maintain a healthy weight reduction.
Therefore, if you use about 2,000 calories per day doing nothing (adjusted by age and gender) and if you went to spin class 3 times a week and burnt 1,000 calories each time, you would use 17,000 calories in a week.
You should eat no more than 10,000 calories (in this example) to safely lose 1kg of body fat.
Myths and diets take over from here and there are millions of different theories, but you can't get away from one simple principle...... to go backwards, you have to use more energy than you put in.
As I said, you'll get out of it what you put into it.
Getting started though, I'd suggest you focus on the cardio fitness first rather than the diet.... you're going to need a bit of help.
Controlling your diet and increasing the excercise (and intensity) will get you results.
Watch the fatties in the class, they will do bugger all and I doubt they are as dilegent with their diet as they let on.
Spin Nil...... SPIN !!!