For AT/Randonee on mountains, you'll need to go up steep (~30°) slopes on skins. You'll need a skin will cover nearly the whole ski base, and that won't slide easily at all (I think my skins require more than 10° of slope to go downhill).
Because of that friction, you will not experience much that resembles cross country skiing, except for maybe some skating type action. Without skins the skis will give you no grip, you'll have to use poles & skating to go anywhere. Ideally your route will not have any long flat sections.
A skill you might work on is going uphill with all that weight on your feet. You won't need to lift your foot for every step, as long as the skis tips stay on top of the snow, so that's just a matter of exercise. You will probably have to change direction on steep slopes, meaning lift one ski out, turn it ~180° then transfer your weight onto it, while your poles are sinking into that bottomless powder. This can get quite awkward and a little frustrating, so I'd practice that before committing to a long day in the back country.
For the downhill part, you'll get conditions similar to groomed snow during the spring, which is easy enough. Depending on where you are you might get deep powder, very chunky sun cups, avalanche debris that's like rubble, or inch-thick ice. You can practice skiing those while in-bounds, if your local ski resort isn't too obsessive about grooming.
As ab2 said, get some avalanche training, and find out about your local avalanche forecast. Get a beacon and practice using it, some ski areas will offer clinics on beacon use. Find out about the weather and don't get caught on a big icy mountain; many people have died from uncontrolled slides. Best of all go with somebody with experience.