The uncomfortable part of the workout is the workout. Everything else was just the warmup.

The point where it starts to feel hard is not a warning, it is the arrival. Everything before that was preparation, comfortable and necessary and not the thing itself. Most people stop right at that boundary, because it is where the activity stops being pleasant, and then wonder why nothing shifted. Difficulty is the ingredient here, not a side effect.

Next session, notice the moment you want to quit and give it two more minutes of good form. Two, not twenty. Sharp pain is a different signal from tiredness, and that one you listen to.