I've had much improved endurance on a ketogenic diet, although I am far from an Olympic level athlete. I burn about 1.6 grams of fat per minute in steady state endurance. 

For mountain biking, I find with a small scoop of resistant starch beforehand (UCAN Superstarch) I have plenty of burst for all-day biking up and down the local ski hill in the summer without refueling at all (no need to break for lunch when keto). I ran a half-marathon in September on bacon and egg yolks for breakfast (zero carb) and tried ketone esters for the first time. I ran the entire race without refueling.

Notably, I've had no inflammation and quick recovery. Two years ago Sami Inkinen and Meredith Loring rowed unassisted from California to Hawaii on LCHF, each doing the equivalent of two marathons per day for 45 days in a row. Exercise scientist Dr. Stephen Phinney measured Sami's blood biomarkers the day after and there were no signs of inflammation--after the equivalent of 90 marathons in 45 days.