How do rock climbers not get tired?

Part of a rock climbers training is to be able to climb for longer periods of time. They do this by doing endurance training, which allows them to scale up very tall walls and long routes.

This also comes in to play with bouldering. Climbers are able to have long 3-4 hour climbing sessions by resting in between routes and attempts. The amount of time each climber rests depends on the climber, their endurance, and the route being climbed.

Generally, it's not abnormal for climbers to spend over 6 hours climbing. It's just the nature of the sport. Climbers are not constantly exerting energy the whole climbing session, they are resting enough time in between attempts to allow them to make the most out of each training session.

Even when climbing a big wall and a long route, climbers are able to find certain spots on the route to rest. And this is where climbers excel. Climbers can find spots to rest mid-route that would seem impossible to non-climbers. These spots could be very small hand holds, but for a climber, this could be all they need to recover some of their energy, reduce the pump in their forearms, and continue climbing.