how the hell does kenma plan to get through 5 set matches with stamina like that?

He'll rest during time outs both of his team and the enemy team's :)

in volleyball normal time-outs are 30 seconds long and technical time-outs are 60 seconds long. even between sets they usually only get a 3 minute break. volleyball doesn't give you a lot of time to rest unless you're subbed out, which is not something they'd want to do with kenma since he's their ace setter and they're at the national tournament.