What should i do to keep my stamina last long?

Question

when i do running, i always keep slowing down.

Answer 1

run longer...increasing the time everyday.

soon you will realize that you can keep running at a steady rate

Answer 2

Your body has natural limits.  Pay attention to yours.  Don't "run thru the pain".   that doesn't work.   You may have chosen the wrong distance to run.  You may be a 400 yard sprinter who gets tired when running 2 miles.  Try all the distances and find your groove.  

Only a very few can become champions.  Don't worry if you fall short.  Running is one of the things that can bring you joy in life for your whole life.

Answer 3

Try out intervals. Run fast for a minute, walk for 2, repeat for your workout. Then slowly increase the running and decrease the walking. If you have a field or track, spring on lap, walk two, or start off with sprinting 100m, walking 200-300m, repeat.