A myth.

Taking regular breaks wouldn't allow an athlete to keep running indefinitely. Exhaustion would catch up with them eventually and their bodies would shut down and force them to take proper rest.

Mark remembered Ramsay teaching him that the Mind stat affects the speed at which MP is recovered and so he immediately understood why he would get a debuff.

Simply put, by continuously consuming his Mind Power he exhausted its natural recovery. He was tired to begin with, and without getting a good night of sleep it was only logical that the speed at which he recovers would decrease.

After learning that much one would think he would drop everything and get some sleep, but instead, Mark kept going, and by the time Lara came over the debuff reached -20.

The entire time Mark was hoping for it to be a mistake. Just when he thought he had found the solution to his Gold troubles, another rule revealed itself and forcefully slowed his progress.