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.
You've got a good point about core training and it taking some load off the legs. When we are tired in one area we tend to 'limp' causing that good area to go bad. So yea , many good points here.