Researchers have not seen any evidence that the ketogenic diet (or any other diet that trains the body to engage lipolysis to a greater degree) increases exercise capacity.

They have seen evidence that these diets can retool the body to more readily use fat as an energy substrate. While this doesn't translate to performance gains, it can mean that on a multi-hour/multi-day effort that a self-supported athlete might need to have a bit less food in their packs as well as foods with a higher proportion of fat that are lighter. Lighter pack = faster athlete.

But...even low intensity exercise over long duration is going to result in the recruitment of FTa/FTb muscle fibers that run almost exclusively by glycolyis. So an athlete counting on fat to be the primary energy substrate has be really highly trained, practiced, and know their pacing thresholds really well to keep the recruitment of those muscle fibers to a minimum.

Best paper I know of on the subject is this one: https://static1.squarespace.com/static/55b7ffebe4b0568a75e33...

Best book I know of on long-duration effort is this one: https://www.amazon.com/Training-New-Alpinism-Climber-Athlete...

According to this [1], running is much more efficient at improving cardiovascular endurance than biking, in terms of time spent training for a particular gain. Particularly trail running.

[1] https://www.amazon.com/Training-New-Alpinism-Climber-Athlete...

(ps. As someone who is sort of a weekend outdoors entusiast, without being any kind of a competitive climber or particularly interested in training, I thought it was an interesting read and would recommend.