Hills are the barometer that tells me I'm getting stronger. When I anticipate a challenging hill on a ride I haven't done in a while and don't recall where the hill is on the route, and realize I just completed it --- and it seemed flatter than last time --- I know I'm getting stronger.

According to the self-assessment in Friel's Cycling Past 50, climbing is my strength. Endurance [currently] is not, but historically it was so I'm working to get that back.