Weight gain when you increase mileage or intensity is absolutely normal! This is not caused by muscle gain, (It takes months/years to gain a lot of muscle, not days or weeks) but water retention! This is normal, and will ease off as your body adapts to the new demands you're placing with.
I also want to echo what others are saying... 1500 calories is NOT an obscene amount of food, and for an exercise program as intense as insanity, YES, you need that much to support that level of exercise, or else you're short-changing your metabolism and slowing it down in the long run. www.sparkpeople.com/community/ask_the_expe