In Connell's "The Most Dangerous Game," how do we know Rainsford is an exceptionally fit man?

Sanger Rainsford is Richard Connell's protagonist in "The Most Dangerous Game." The reader first discovers Rainsford is a fit man at the beginning of the story after he falls off the yacht and swims through ocean currents toward Ship-Trap Island. The text says the following about this swim for safety:

Doggedly he swam in that direction, swimming with slow, deliberate strokes, conserving his strength. For seemingly endless time he fought the sea. He began to count his strokes; he could do possibly a hundred more.

From this passage, the reader learns that not only does Rainsford know how to swim with intention, but the swim isn't easy because he has to fight through the ocean.
Rainsford then proves he is at the top of his game physically when he is hunted for three days in a jungle. For instance, Rainsford runs through the forest, climbs trees, digs pits, and makes elaborate traps during this three-day traumatic activity. Not only that, but he does it with very little sleep. 
Additionally, Rainsford proves he is in top shape when he jumps from some island cliffs, survives the fall, and swims around the island at the end of the third day. He then finds his way to General Zaroff's personal quarters. Zaroff is taken completely by surprise as follows:

"Rainsford!" screamed the general. "How in God's name did you get here?"
"Swam," said Rainsford. "I found it quicker than walking through the jungle."

Rainsford does not die of exhaustion, nor does he get injured over the course of such an intense few days. Only a man in the best physical shape could have accomplished what Rainsford does.