New to Cycling: Struggling with Endurance. Looking for advice on refueling and other strategies.

I’m new to cycling, having started in March. I’m 56 years old and weigh 230 lbs. My typical ride is 25-35 miles with about 700-800 ft of elevation gain, and it usually takes me 2 to 2.5 hours.

Some days, I feel like I could ride for another hour, but other days, I struggle to get through 15 miles. I know part of it has to do with how much sleep I get the night before, but I’m wondering if my refueling strategy could also be a factor.

I’m working on losing weight (let’s set that aside for now), but any advice on nutrition or fueling or other strategies would be hugely appreciated!

I want to consistently do 50 miles on the weekends.

As a side note: I have a 2022 Trek Domane SL6 with SRAM AXS (Rival).