The average lean male human has enough fat to run several marathons, if he has trained himself to use fat as a primary fuel. Might not be the fastest in the sprint, but that's how you ski to the pole ... and get back.
When properly fueled, we have about enough glycogen for approximately 90-minutes of very hard exercise.
We have ~ 80 times that energy availability in our fat stores (more if you are fat of course).
One doesn't train how to use fat as primary fuel, the body already knows how.
What we actually do is to train to improve our aerobic capabilities (e.g. through increased capillarisation in the working muscles, increased mitochodrial density, reduced diffusion distance for the supply of key metabolites and gas exchange into the muscle cells) as that lifts both the absolute and relative power output at which we still utilise a significant proportion of fats as fuel.
When not glycogen depleted, then the fuel substrate utilisation ratio is more a function of relative intensity (for example, % of VO2max, or % of maximal aerobic power).
Ironically, the most effective training to improve one's sustainable aerobic abilities and use of fats at higher intensities are harder aerobic sessions that rely almost exclusively on glycogen for fuel.
We use glycogen all the time (even at rest - our brain can only use glycogen for example). Here's the typical fuel substrate utilisation based on exercise intensity (% of VO2max):
Pic from
Textbook of Work Physiology - physiological bases of exercise, by Astrand and Rodahl
