How many chapters before Reona falls for Saotome? I'm saying 5 or less.
We're already in the mountains, so I foresee one of two scenarios:
1. She rolls her ankle/cramps due to overexertion on the final stretch of the run while head to head with main girl and can't keep running, so MC picks her up and carries her over the finish line
2. Everyone arrives at the goal, but Reona isn't there because she slipped down a ravine while trying to take a shortcut (because she was too eager to win, obviously), and is currently being threatened by several bears, so Saotome saves her
She'll also lose part of her clothing in both scenarios and shove her tits, ass, and/or vag into Saotome's face by accident.