How can I stay full of energy while working landscape. Will my body adjust to it? (1st day)?

I started working landscaping again from last summer today and I'm flat out exhausted. I can barley walk. We lay sod, do sprinklers, level the ground, and a ton more. I'm only 15 and want to know how long until my body will adjust to this because I'm going to do it often this summer. Also, how can I keep myself from getting tired soo quickly. Thanks