Hello goatigirl, you've already gotten good replies, but I would like to echo some of these thoughts. If I was having difficulty with my legs, I would do as Shawn suggested and have a chair right behind me. I will also suggest an alternate to sitting down. You can straighten your legs a little (not to the knee lock position) and it should be easier to stand.
Sitting is fine too. It is all part of the Good Better Best idea. I would suggest completing the active meditation in the modified stance or seated position. Modified meditation is better than no meditation. If after sitting out one or two movements you can return to standing, that too is better. Complete the meditation as best you can.
As you go from best posture to modified posture keep a positive smiling and grateful mind. Be thankful to had done so much and soon to do more. Any negative thoughts about your practice should be smiled at and sent on their way like willful children.
You are perfection,
Iam2
