"The first half day is pretty painful" ...
Dude man (as a buddy says when he gets exasperated
, I don't know why every snowboard instructor/school doesn't loan first-time students hardshell kneepads ($10 at Home Depot), a thick tailbone pad (you can make dozens out of a 5/8" ensolite pad, approx 12" squares, or wider to cover the hip bones), wrist guards, and a helmet. Since you already have a knee issue, and maybe aren't in your 20s any more (skiing 25 years
, do yourself a favor, and wear a full set of pads when learning to snowboard. I wear all that stuff every day, and I've been snowboarding since 1984 (maybe not in my 20s any more either
. I tend to go fast in trees and the steeps. I know a pro skier/photog who wears a full motocross suit under his ski gear; loves it. Have also heard of guys using hockey pads, football lower body pads; whatever is light, comfortable and priced right. There are loads of padded shorts for sale these days, but I have not found a comfortable set that seems worth the price.
I have had knee and ankle issues from skiing. I have never gotten hurt in the powder when snowboarding. On hardpack, I have hurt my wrists, esp. thumb joint. Even a simple neoprene wrist wrap, or padded mtn bike gloves under your mitts, helps the wrists alot. Stay in the powder; even the heavy stuff.
Be careful getting off the lift if you have one foot on the board. That could be hard on a knee. Make sure you have a grippy stomp pad for the other foot. Detachable quad lifts, or a gondola are sweet for snowboards. Or just hike for backcountry powder; you will never hurt a knee that way