Do I change the weight on the leg press machine???

I am pre-surgery and started working out. I work at a hospital so I have free use of the Physical Therapy equipment. Employees just have to let the patients use the equipment first if needed. 
I have bad knees(due to weight) and they started me out on the leg press machine with 3 sets at 60#'s. Well I felt that was too easy but did that for almost 2 weeks for 2-3x a week. I increased to 68, then 72 and now 80. I can do usually 4 sets and just did 6 sets last Friday. I stopped because I knew I was going on the tradmill next and I didn't want to pu**** too much.  Sooooo long story short. Do I increase the weight until I can only do 2-3 sets or continue with what I have? The therapists really aren't there to ask and they are busy with patients.

When we are heavy we have strong legs.  It has always been my opinion that keeping strong legs is very important in burning calories since muscle burns calories at rest.  I would say that yes, you should increase the weight.  Good luck with the surgery.

Thanks