Snowboarder with tendonitis

I was a gymnast for many years as a boy but i had to give it up at the age of 14 because i had developed tendonitis. After giving it up, my knees got better. However i was on a snowboarding holiday a few years ago and my right knee really flared up and it was pretty painful – the snow was slushy and it was very chopped up so i was using my knees a lot. I had to stop snowboarding for the rest of the holiday. I have been on a few holidays since and it has been fine but i am spending the whole winter season in whistler this year and i’m concerned that my knee wont survive a whole season. Are there excercises that i can do to build up my knee? Any advice would be great.

Cheers