This year I worked out all summer to get back in shape. I dropped the 20lb excess I had gained during my wife's two years of pregnancy. I still have a beer gut somewhat because I love to eat and drink beer, but I definitely strengthened my core.
I do martial arts on a heavy bag. Right now I'm just shadow boxing because my bag is outside and it's too cold lol. Kicking and punching for an hour or more at least three times a week.
I know you only mentioned legs, but I'm bringing all of this up because a total body workout along with cardio increases your stamina all around and makes you a better rider. I was very out of shape last season and I definitely notice the difference in my riding this year.
Core workouts are the most important and will also reduce the fatigue on your legs. If you have a strong core, you're more balanced thus taking some load off your legs.
For my core, I do an ab wheel exercise everyday. It took a month for me to be able to do it everyday and to be able to stretch all the way out and down. Such a simple tool and cheap investment for something that stregnthens your core so quickly and effectively.
The cardio you can get from kickboxing is awesome. Even when you shadowbox, you get a lot of cardio so long as you're still putting power into your punches and kicks. You'll get a leg workout from this as well as a core workout. On top of that, it's just plain fun and stress relieving.