Ok, one thing at a time here. Pain isn't always good. Ensure you have the bike set up properly, ie, saddle height and angle, bar position etc. Call in at a good bike shop and ask their advice. Next, calories. Websites can give a general idea, but they can be quite a way off. Calorie burn is specific to the individual and varies with fitness. Use the website as a guide, but don't rely on it. Some heart rate monitors also calculate calories burnt, by using a test to calculate an approximate VO2 and then using your HR to measure a personal effort level. These are better, but I wouldn't say perfect. Calories, part 2. Have you considered part of what you burn is from your glycogen stores. If you exercise hard, these are used first, then fat reserves. If you are going steady, they are combined. Don't go flat out, you should be able to talk, albeit while breathing reasonably heavily. Add in more calories to your calculation to cover this. Lastly, the bike. An mtb is fine, just consider some slicks if it is all surfaced roads, this will make the effort a little easier. You may decide to do some offroad as fitness increases. The position is also more comfortable for someone just taking up cycling.
Oops, nearly forgot, don't forget to "spin", that is pedalling at somewhere around 70-90 full pedal turns per minute. This eases the effort on your muscles, while making your cardiovascular system work. You'll ride further with less fatigue and faster recovery, while still burning lots of calories. Now get out there and ride!!!