Re: TKR in 8 weeks . . .
Hockeyknee - I had a TKR on my left knee on 12 Feb, 2013. I worked out like a maniac for 3 months prior to my surgery. For Cardio I did the elliptical, and for weights I did machine based leg extensions, leg curls, seated calf raises, hip adduction, hip abduction. I also did a lot of core stuff as well as triceps, bicep curls, lat pull downs, and military presses for upper body.
Looking back on my recovery I would have to say "yes", getting in great shape really made that first couple of weeks a lot easier then if I had not. For the legs, it wasn't the surgical leg as much as the other leg. After surgery, your quad/hamstring are incapacitated for several days. Having the non-surgery leg in great shape helps you get around much easier. When it comes to upper body, having your core and triceps fit really helps with the walker and overall mobility etc. Also, being in good Cardio shape just insures you won't develop any respiratory problems.
Hope this helps, and good luck with your TKR.