Haven't been able to find exact specs for ketones, but they are faster than mitochondrial fat and slower than mitochondrial carbs.
A good bit of creatine phosphate is backward restored by freeing the Pi from mitochondrial ATP - doesn't matter if it is lipid or carb based. If you don't have a ready supply of pyruvate, all or most of your mitochondrial ATP recharge will be from palmitate (lipid). Is based on local supplies, so lack of muscle glycogen is a significant factor.
You can improve recharge via steady state aerobics or HIIT, which will help increase number of mitochondria. Their ability to process fats for ATP is pretty much reaction based and doesn't change a lot from sedentary people to athletes, is quantity limited.
Training at higher lactate threshold creates changes in mitochondria that are qualitatively improved relative to processing pyruvate for ATP - the reaction/recharge time shrinks relative to sedentary and folks who don't train at higher intensities. This is why for higher intensity steady state athletics, carb loaded folk tend to dominate