I turn 65 in October. Each winter my quads take an early season beating. Not so bad on skis but snowboard racing turns 'em to jelly. This is the best exercise I've found for the off season.
I experimented for a half dozen years alternating between intense off season workouts on the leg muscles and doing essentially nothing. Each winter the quad of the forward (left) leg turned to jelly the first few times out. Unicycling does the best job avoiding the problem. One other change I implemented last year did seem to help. I canted upward the toe of the front binding and the heel of the rear binding. The photo below is of the earlier flat configuration but shows my board is far from traditional. This one is 178cm. My ski racing isn't impacted nearly as much as my snowboard racing.
Hills? Actually they're easier to climb on a unicycle than to descend. Trying to keep the uni from getting away from you on descents takes a LOT of leg strength.