On a ketogenic diet with adequate protein, creatine stores and full glycogen stores in muscles should prevent any decline in exercise performance.

“if you're expanding you know 80% of your vo2 max you're gonna run out of glycogen in about 90 minutes if you're expending 120 percent of your vo2 max you're gonna run out of glycogen in you know 45 minutes it just depends on just how all-out you're going but if you look at every elite marathoner on the planet they will all balk at about two hours no matter how fat adapted they are you will literally deplete every bit of glycogen in your liver and your muscles and you cannot maintain glucose high enough just to really even stay conscious”