A stiffer board would usually work out the leg muscles more. Though my camber board (Burton T6)... it would take a few hours for fatigue to set in (unless I'm cruising greens all day). My flying-V (Burton Sherlock), unless I'm hitting moguls hard, doesn't wear me down as much.
so, when you're hitting the gym doing squats, it's not about maxing out... hit it a little lighter and go for a lot of reps.
Biking is great conditioning for the legs for the snowboard season. Mountain biking (technical riding) is a great total body workout. Road biking is great for conditioning.