1st timer - welcome!
I don't think the slimfast thingy would be a good idea - you are burning loads of calories with the amount of training you are doing, and not getting enough energy and nutrients to recover from your sessions will leave you knackered and open to injuries.
www.realslimmers.com have some calculators which give an indication of how many calories you burn just doing normal things, and for specific exercises. They will never be totally accurate, but give a useful guide. Given your weight, which is slightly less than me, if you have a desk job, you will burn about 1600 calories a day just existing. Add roughly 100 calories a mile for steady running, and 2-300 per hour of body-combat/pump depending on how intense the class is. het will give a rough guide of what you are burning. Try not to consume less than 200 calories less than this per day, and you will still be losing weight, but at a rate which will be sustainable, and will have the best chance of staying off.
