I think the only way to build up your endurance is to run farther each time. Run as long as you can, making sure you note how long you ran for. If it was 10 minutes for example, for the next week or so run 10 minutes. The following week try and run 12 minutes, and then the week after that add on another minute or two. You can also do a few seesions where you run for 5 minutes, walk for 3 minutes, run for 5, walk for 3, etc.
When I first started running on the treadmill I could only go for 5 minutes...it took me awhile, but I finally managed to get myself up to 30 minutes. I find the only way for me to go longer and faster is to push myself...but not so hard that I am in deep pain and stuff. Good luck!
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