Welcome to HealingWell, chrissy. I'm just a bit older than you, and did have a knee replacement three years ago. I hate when doctors tell patients they have to lose weight before they can have surgery. I'm not sure if the surgeons are trying to improve their success percentage or if they're punishing people for being overweight.
At any rate, there are exercise regimens that can be done seated. These have been developed largely for people with arthritis who have limited mobility. One good instruction can be found at this site:
http://www.livewellagewell.info/study/2007/12-ChairExercisesUGA113006.pdf
This is an adobe file; if you can't read it, try Googling "chair exercises" and you will get links to other sites.
I've actually found that non-weight bearing or low-impact exercise helps relieve my arthritis symptoms in other joints. It seems strange, but I know I will hurt much more if I miss my workouts for a week. Be prepared to have some muscle soreness at first; that's normal when becoming more active.
Good luck, and keep us posted. I'm sure others will have additional suggestions for you.