Given some of your physical issues you might want to consider going with a recumbent. You certainly won't have to deal with the pain issues of lack of flexibility in some of your body parts. (Sore back, neck, hands, and butt) The knee thing should not be an issue on any bike as long as you don't overstress them (spin in an easy gear, don't mash). Some of the clipless pedal systems allow for "float" which should mitigate the foot alignment problems. You wanna go fast? Look at the RANS F-5 or Bachetta Aero. These are basically road bikes with the recumbent riding position. The Bachetta RAAM team was third overall this year in the four man team division riding Bachetta Aeros. These guys had an average age of 42 compared to the fair bit younger teams they were up against. So (some) 'bents can go fast. As long as you are not planning on doing any competition sanctioned by UCI or run under UCI rules a 'bent is legal. Incidentally RAAM was over 3000 miles and had 110,000 feet of climbing, and these guys averaged over 20MPH on that race. Just some food for thought here is all.