Cycling
 
 
In 2006, a cousin of mine convinced me to ride with her from San Francisco to Los Angeles...  545 miles in 7 days...  Needless to say, I learned a lot about myself and riding in those 7 days (and the months of training ahead of time).  I had never used "clip-on" shoes before, but for those hills, it's a requirement.  What I learned is, they not only double your workout, they lock your feet into perfect alignment for your ankles, knees and hips.  They double your workout by working your hip flexors, along with your quads.  But, I think the effect on your ankles, knees and hips is more important.  It works them out in perfect alignment without any impacts.  It's perfect training.
 
I believe all stationary bikes should require clip-on shoes, for just these reasons.  You double your workout and over time your ankles, knees and hips re-align to where they are supposed to be.  And it gets you used to using them before trying them outside on a real bike under real world conditions.
 
I try to get in 100 miles a week on my bike.  I don't always and I don't ride in the winter.  But I try.