dude i've heard of steroids being used by athletes to go beyond their own potential limit, such as strength, size, and speed. and even then they work their asses off much more than the average person not on steroids.
but what ure asking for is a shortcut to get endurance? stamina? i dont knwo of any athlete that uses it for endurance (to run without getting tired). endurance comes through practice, practice, and practice. the more u run the more endurance u get. everytime u get tired u push ureself beyond that barrier so the next time it becomes easier. no steroid is going to give you that break through without u breaking through ureself. give me a break.
get ure ass up and run even if its for 10 minutes only, atleast u did it till u couldnt lift another leg. keep running everyday, day after day up hill up stairs, light squats once a week, and keep running. then u'll get great endurance.