Dear Klizark,
Good for you for resolving to do the SFQ on a regular basis. You are doing a good job. When your knee gets tired, you might try sitting for awhile, and continuing that way. Master Lin says the these exercises can be practiced standing, sitting, or lying down.
The 7 Steps of New Life is challenging for people who don't balance well. At the Level 1 class I took, when we learned that one, we all were sitting down in chairs doing it. That may be because the room was quite crowded, but for other exercises we did stand up. So, it's perfectly OK to sit if you need to.
For people who are really sick, they can do things lying down and still benefit!