single lub wiring in sub panel.

Is there a problem with only wiring one leg on a sub-panel? I have a metal shed with a UF 120v (#8/2 w g) on a single pole 40A breaker running from a pumphouse 40 ft away. Previous owners never hooked up the wire in the shed, and now I would like to install three circuits, one 15A for led lights, one 20A for a couple of outlets, and one dedicated 20A for a small AC unit. Any problem wiring only one side of a 6 breaker box (115v line) ?