Those calorie counting thingers on exercise machines are a bunch of hooey... how many calories you burn doing an activity varies widely from individual to individual. Heavier and also leaner people use more energy doing activities than smaller, thinner people.

When I was your weight, I couldn't do more than 5 minutes of any moderately strenuous activity at a time before feeling faint. However, unlike most people, I was also carying around 100 pounds worth of weights while doing it. Don't feel bad, it's totally normal to not be able to do a lot of exercise when really overweight.

Now I walk everywhere, and I don't even "feel" it. Though still pretty out of shape, I can do 30 minutes of walking/running combo cardio and still have plenty of energy.

As you lose it will get easier, so hang in there!