A few comments and questions:
1) What is your current weight and fat percentage (NOT body mass index)?
2) You should be tracking your fat percentage, not body weight if your goal is to "thin down"
3) 1500 calories sounds extremely low... your body is probably burning muscle to fuel your long rides. If you are really riding 40 miles -every- day then that calorie intake is super-low.
4) How are you counting your calories and determining how much you burn on each ride?
5) Once you are 100% positive that your diet is appropriate for your activity level and weight loss goals then see if you are dropping fat percentage. If not, try shocking the system with a different form of activity (swimming, weights, jogging, etc.).
In order to burn fat you need to have a -small- caloric deficit. Your body will not go into starvation mode (which -conserves- fat) and it will allow you to burn fat for energy. Now obviously nothing is absolute... you will burn some fat no matter what but you should be able to optimize your results.