I am 64 years old and my average long distance speed is 14.5 -15 mph. On the most recent cross country ride I did (New Orleans LA to Itasca, MN along th eMIssissippi) took me through flat hot and hunid delta riding, rolling hills and wind, steep climbs with and without wind. Since I wear a body monito I can safely say that on a average day ( 75 miles) I burned between 3,000 and 4500 calories above and beyond the 1100 that is my daily burn.

that pretty much matches the average burn on the three previous cross countries. Riding around locally where it is flat and somewhat windy, I averagebetween 300 and 400 calories an hour depending on the head winds and how hard I am puess shing to maintain 15 mph or above.

I guess what I am trying to say is that if you know you are going to ride long and strong, you need to prehydrate for a day or tow ahead of time, which for me is daily since I ride 50 miles 24 times a week and do cardio 2 x 2 hours a week. I tend to eat whole foods on a fairly regular basis. On my trainig rides I consume a breakfast of oatmeal with dried fruit ad nuts and a banana before the ride, a banana, a cup of dried fruit and nuts, and a couple of gu gels. I carry one bottle of FRS and one of plain water After ride I eat a chocolate almond recovery drink, a sandwich of whole wheat bread, meat and cheese, a large salad and a piece of fruit.

I also like Like bars which are basically dried fruit, nuts and honey and carry one or more of those as a back up on longer rides.

Keep experimenting and you will find something which keeps you going but the longer the ride ad the harder you push, the harder it becomes to not build up a nutrtion deficit for the day.

One other totally random thought... If you are bound on increasing distance, wisdom says that you shouldn't increase more than 10% a week so if you do 50 one week and 75 the next, you are pretty much going to feel it.