I tend to associate not hydrating well with prolonged tiredness and fatigue after a ride. But it sounds like you hydrated reasonably. I also tend to feel that under consumption of carbohydrates during the ride also contribute to that same feeling to a lesser extent. But you need to make sure you are getting the carb's you need both during the ride and immediately after.
So how many carb's did you consume while on the ride? A 2.5 hour ride for me will have me consuming 300 to 400 Calories. Almost exclusively what I put in my bottles. I seldom eat anything while riding. When I get finished, I usually get another 300 Calories, mostly carbs and some protein fairly soon after the ride.
I also don't worry much about a "pre-ride" meal. Riding with too much in my stomach has always seemed more detrimental to my performance than not having anything in my stomach.
Mostly it might just be you are doing too much too soon since you said you just started back. Also, if you are riding in a group trying to keep up with them, then you might be over reaching at the moment and fully exhausting yourself just so they don't make fun of you. <grin>