So if you're "good enough" you can train your cells to use something other than glucose to produce ATP?

Your cells already know how to produce ATP from fat. (except for brain cells, and maybe a few others) there are more steps involved than the glucose pathway, but the cells already know them. All you have to do is up regulate those pathways in the cells that already have them. This is why your body makes/stores fat in the first place: it is a nice way to store a lot of energy for later use that doesn't have the problems of excess glucose in the body, if the body couldn't turn it into ATP it wouldn't bother making fat.

Ask a real biologist for details. I know this was covered in my high school classes 30 years ago, but I haven't studied it since so I don't remember details or know anything later research has discovered.

A ketogenic diet can cause human metabolism to shift from glucose to ketones for power consumption.

My memory on this is rusty but I recall fat being usable as fuel by the body after being converted to ketones.

That is only for the brain. Muscles can use fat directly for energy. Glucose is a lot easier process and so the body will use that first. If plenty of sugar is always available your body won't develop the fat pathways as much.

This is not good for your liver long term: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7008768

Keto causes fatty liver disease

That is a part of what long distance running and cycling training accomplishes: shifting your consumption to a higher mix of fats vs glycogen at the same intensity level. (To avoid the pedantic "buts" here, yes, your body is still consuming a lot of glycogen when you're running a marathon, but it's also using fats at a much higher % than it would in a 400m).