So there are a few things that contribute to a rising HR in relation to watts.
There’s heat, which increases the need for blood flow to remove the heat along with the fact that as you get warmer your body tends to use more carbs, I’ve seen some literature that as duration increases as well as heat and intensity, it can lower your LT as you ride, thus leading to greater cardiac drift. With an effort that length, and with the varying intensity there are a few things that could help. Training to improve aerobic fitness (here) as well as improving capacity to switch between using different fuel sources (fat and carbs). Often when you ride hard you switch to carb usage but it can be difficult for the body to go back to burning fat. Again why the HR can stay elevated compared to power
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