From my personal experience as well as from hearing other people's opinions and observations, stamina should increase gradually over time with proper diet and exercise
People who are obese have the worse amount of stamina and those who are active in their lives seem to have more stamina
"Judging from what you said, I think you aren't pushing yourself hard enough."
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Just a generalization, your stamina should be increasing over time, as your body gets used to the movement of running or any cardiovascular activity. You will eventually become faster and more efficient at doing so.
My solution to you is :
you should begin to time yourself, what is your fastest mile? try to beat your mile time gradually over weeks. Since you are experiencing your runs to be slow, you should start slow, and work your way up. Whenever you run, you should have a strong-mindset of your goal, which is to beat your previous times
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
I think with proper motivation, you will become faster.