What should i do to become a better offensive lineman?

ok im 14 5'9 265 a freshman in high school this is my first year now what should i do conditioning wise so i can run longer?, cause i can barely make it around the field.when we go to the sled(the thing the coach stands on and we push it) im usually moving it the farthest. but coach dosen't want to play me cause i wont last long in-game and what are some good techniques to know to be a good lineman?