The upper part of my back calf muscle hurts when I skateboard. Am I just out of shape?

No. Skateboarding like snowboarding requires riding with pressure on the front part of your foot during toe turns. That requires your calf muscles to work overtime, thereby causing pain. Try riding more flat or doing heel turns when the calves hurt. Strengthen calves by standing and repetitiously raising heels off ground as high as possible, and then repeat to tolerance.