If your legs are week how are you supposed to sprint and have quick movements through a 3gun stage? If your legs are week how are you supposed to hike for 8 hours during a precision rifle match? Power in your legs and endurance in your legs are both so important in most shooting sports and most sports in general. Think skiing and tennis as a couple of examples. Matches are long and I want to feel just as strong at the last stage of the day as I did at the first stage of the day. Strengthening leg muscles is key!
I have a love hate relationship with stairs but what a great way to get in shape. I have several trails with long sets of stairs near my home. Every morning Kozmo, my pooch, and I hike those stairs. I usually combine a couple of sets with a long walk but some days I will focus on just sets. As many as I can until me legs fall off. I have added my precision rifle backpack I use at matches that is holding all of the gear I would have with me. It is not super heavy, maybe 25 pounds or so depending on the day, but it ads to the training exercise.
My point is, again, get off of your tush and get out there!