You volume is pretty descent.
I'd 1st work on pacing? I suspect your going out closer to tempo on long rides, then power trains off, but the damage is done. that's normal if using RPE.
Since you don;t have a power meter, HR is the next closest thing, but it will start ot drift upward at the same power output during the ride, more if it's hot outside, so it can be hard ot use. It also reads really high after T1 in a race. Speed is hard ot use as wind and terrain changes. If I used speed on a "flat" course like Steelhead, I'd vary from 50% to 150% throughout the race and blowup and that without including the bigger hills. Breathing rate isn't bad, but it too will increase later in the bike ride.
As a example, I did 2 5 hour rides each of the last weekends. Ride #1, I pace very evenly and actually did negative splits by power, riding 5% higher the 2nd 1/2 than the first half. I felt great.. Ride #2, I had forgotten my shoes at the meeting point, drove how and decided for fun to ride hard for 1 hour to catch up with my riding partner who was riding with a slower group in the meantime. I rode about 83% average that period. I then felt OK for the next 1-1/2 hours, rode hard for another hour, but the last 30 minutes, I fell apart. Average power overall was lower on this ride. Nutrition was the same.
Nutrition? You sure your eating enough. Lack of nutrition feels, in my experience, almost the same as leg fatigue or just blowing up (lack of metabolic capacity for that duration)
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