Tried playing for 1/2 hour. With all the arranging and tuning, ended up being 10 minutes.  My thumb joint initially hurt when I bent it after plucking a string.  However, that went away about 5 minutes into playing. However, however, that could be due to my usual "pain in hands goes away when I play". 

I tried closing my thumb more like the left one, ie not aiming the tip of my thumb at the 1st finger knuckle, but the thumb knuckle to 1st finger 1st phalanx/knuckle area.  Kind of like when you make a fist puppet.

I am hoping that I am not just paranoid and that the cyst does not look bigger after playing.

So my goal is to play 1/2 hour every day for a week then bump that up gradually as I get back into shape. I will not be playing Prelude 1 again, though, that's for sure.