Assume burning approx. 500-600 calories per hour ... so on a 6-7 hour ride that would be 3000-4200 calories.
Consume 200-300 calories per hour ... approx half what you burn. If you have been eating normally in the days before the ride, you should have approx. 2000 calories in storage to use. Between a good 500-600 calorie breakfast, your 200-300 calories per hour, and your 2000 calories in storage, you should be able to last 6-7 hours just fine.
When rides start taking longer than that, you may need to stop for larger meals to catch up on your calorie needs a bit. After the first 6-7 hours, I find I need to stop every 5 hours or so to eat a meal.