With pending knee replacements, you may want to consider a step-though frame. I started riding again on a Jamis Citizen 1 step-through after my hip replacements. The Jamis Coda S3 Step-Over would be similar to what you have, I imagine. The Trek FX 1,2,3 Stagger models are actually pretty light.
As for Rivendell, several suitable Rivendell bikes are currently available only in Small size. The Clem Complete is currently available in 45cm and 64cm. I like my brother's Clem Smith Jr. - he bought the 64cm frame and had me build it up:
Rivendell Clem Smith Jr.
It's a great 'hop on and go' bike. It is really long though, so not suitable for tricky trails. Great for roads, though, gravel or paved. Now with 35mm Continental Terra-Speed tires. We'll mount 48mm tires on his new wheels when he gets here.