How do we know Rainsford is an exceptionally fit man in "The Most Dangerous Game"?
After falling off the yacht, Rainsford shows his athleticism by surviving the fall and the long swim past the treacherous rocks to Ship-Trap Island. He began to doubt whether he could make it.
For a seemingly endless time he fought the sea. He began to count his strokes; he could do possibly a hundred more...
But when shots rang out in the distance, Rainsford renewed his efforts.
He was almost on the rocks before he saw them; on a night less calm he would have been shattered against them. With his remaining strength he dragged himself from the swirling waters. Jagged crags appeared to jut up into the opaqueness; he forced himself upward, hand over hand. Gasping, his hands raw, he reached a flat place at the top.
After a comfortable night in General Zaroff's home, Rainsford was forced to be the prey for the Russian's special game, and Rainsford was used all of his physical reserves to survive his two nights on the run. He began by moving quickly, then executed "intricate loops" to double his trail. He crawled when he needed and got no sleep the first night. The next day he climbed trees, set traps and survived a brush with the quicksand in the Death Swamp. Realizing he could not evade Zaroff's relentless pursuit, Rainsford chose to dive from the cliff and take his chances with the rocks below. Another long swim returned him to Zaroff's chateau, where he waited in Zaroff's bedroom. And then it was off for another round of hunting--this time with Zaroff as the prey. Rainsford's physical prowess no doubt contributed to his being the winner of the second hunt, and he was finally able to enjoy a comfortable sleep in Zaroff's own bed.