If you are heavier than the other riders you will have to put out a lot more watts to keep with their pace on the hills. If you are behind on cresting the hills your added weight will give you a little advantage but as your speed increases that advantage is minimal, so you again need to put out a lot more watts to catch them. On a long ride this just adds up and you totally fatigue the muscles and you get no rest not to mention fewer opportunities to draft, no amount of fuel or electrolites will bring the muscles back - only rest. Hermes is correct, you need to train to get stronger if you are going to hang on these longer rides and if you are heavier than the guys you want to ride with and it's a hilly ride you need to be stronger than they are or else you need to match their weight. Gravity and wind resistance are cruel teachers.
Now on the hydration front, IMHO you were way under hydrated - 4 bottles would be more like it. You may have also been under fueled, two Gu shots are only 100 cal, you should have put in about 300 at the 30 mile mark and another 300 at 45 miles to get you home. the gatoraide and scone may have made up the difference but I doubt it.
On a long hard ride I usually have to force myself to hydrate and eat - but once I learned to do that it really helped.