Lets look at human muscle physiology from the viewpoint of being untrained.
Your slow twitch fibers permit endurance. If you have large muscle mass, you might have more slow twitch fibers in the mix, to share the burden, and allow you to run for a longer time. High volume at moderately easy running will build slow twitch fiber mass, and develope better efficiency and endurance, more capillarity and muscle metabolic adaptations.
Your intermediate twitch fibers have some endurance, and gain even more after some high intensity workouts, but naturally revert to mere fast twitch functioning if you only run slowly, and so they don't get called into the game; or if you train too fast, too anaerobically, too often. You build mostly slow twitch endurance over a few months or years of base training, and then enhance that endurance by harder or longer workouts that recruit and develope intermediate twitch fibers (to convert them into more oxidative functioning, like the slow twitch fibers). So now you've added more slow twitch muscle mass, and more intermediate muscle mass, and the intermediate muscles help the slow twitch muscles out, and you have lots of speed endurance.
Then, unfortunately, if you get all enthusiastic about getting competitively faster by training even harder, you end up detraining those intermediate twitch fibers, and they slowly revert back to fast twitch functioning, and you lose endurance. You plateau or regress. Still pretty fast, but not very far.
As another poster has said, just run some distances and see where you are at. Then judge weather you need to work on more endurance, or more speed.
If you are active and young, you might easily use your few slender slow twitch muscles to run far (if you are skinny and don't run too fast). You can train for even more endurance to build up more glycogen, mitochondria, myoglobin, oxidative enzymes, etc.
If you are blessed with an extra quantity of intermediate twitch and fast twitch fibers, you can expect to have better speed, without even trying, but if you train too fast right away, you are likely to tear some of your slow twitch muscle fibers up, along with your faster twitch fibers, or your connective tissues, ligaments and tendons. So natural speed doesn't mean you are bullet proof. Naturally fast runners still need a gradual training plan to allow the muscular skeletal system to catch up.
Most people are not that different than others, and your gifts might be marginal. So, training can take you either way. You don't have to be worried about fighting your genetics. The epigenetics are more important. Exploit your gifts if you don't have time to build up from your weaknesses. Otherwise, do what you like to do. What kind of healthy animal do you want to be?