Their pace had slowed down on the second lap, but the Dragon King actually helped them with food and water so that they could focus on running.
Nathan and Selena started losing the feelings of their legs once they finished their third laps.
On the one hand, they wanted to sleep to recover their stamina. On the other hand, they might not be able to move their legs the next day if they did so.
Only after the Dragon King insisted did they finally sleep. And it didn't even take them that long to fall asleep.
Unbeknownst to them, the Dragon King used some herbal ointments and medicines to help them recover.