I have to second Machka.

I've been doing 3-5k (miles)/year on the bike for several years.

Last year, some friends came to NYC for the first time.

We gave them the 3-day downtown/midtown/uptown tour, walking all day.

A whole different group of leg muscles were absolutely aching for days after.

Conclusion: good core lower-body strength requires low-impact, load-bearing exercise (i.e., walking/swimming) in addition to cycling.

Have been trying to walk 30-60 minutes/day since, in addition to on-the-bike training.