The more the tyre wears, the flatter it becomes and the more surface area is in contact with the road, and so there is more rolling resistance.

That being said, there's no such thing as sustained athletic peak performance. Any decent competitive athlete understands that you can't maintain peak conditioning forever, and athletic training for everything from running to fencing is designed to ramp up to a competition. I'd say the OP needs to take it a bit easier every once in a while, and that such fatigue is part of the natural cadence of exercise.