It takes me at least an hour to get properly warmed up. I think it's all about the generation of nitric oxide in the endothelium. After a while, our hearts don't have to work as hard to pump the same amount of blood. Thus my guess is that you're measuring "hard" by power rather than HR. I've had long hard Z5 sessions at the 100+ mile mark on a mountain ride, which I simply couldn't have done near the beginning of the ride.
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