I can cycle 1000 km months including 500 km in one weekend ... but if I ease way back on my stair climbing for a few weeks in order to do that, and then resume my stair climbing, I'm stiff and sore for a week.

And any time I run on the treadmill at the gym, I feel it the next day.

Cycling doesn't use all 4 quads. At most it uses 3. So it's good to cross train to use all your muscles.