Stilly River Sage, just to reassure you a bit, a dear friend had BOTH knee joints replaced simultaneously (most unusual, as they normally do one at a time)
She found it slightly painful afterwards while the 'work' was healing, but nothing too unbearable. The physio encouraged her to flex and move the knees as soon as she could, to prevent adhesions.

After she was discharged from hospital, I went to her house and stayed for a few nights to help her get to the loo etc, as she had no family for support, and I was quite surprised at how soon she was getting about.
Now she never shuts up about how great it is to be able to whizz around pain-free, and tells everyone never to fear the op.