well... a lot of the triatheletes.. doing 112 miles are about 14-17% bf at time of races.. i think its the running that takes it off..
but if you look at the top women cyclists.. they have a certain amount of muscle too.. upper body strength will help long distance as well as sprinters, so if you have muscle, and strength train, chances are your body fat will not be higher than 20-21%... average training i mean
if you lose too much too fast.. you will lose muscle also, fatigue and over training will do you more harm than good.. a good balance of protein, carbs and fats with 5-6 meals a day will get you on your way and you'll have energy for those long rides.
i ususally do rides on the weekends of 100-300 k.. depending on my time. i am working up to longer rides now and then.my bf is 17%..