We are two retired gents. I am age 74, 6 feet tall, 240 pounds (or so) with a herniated disk (L5). My friend is 6'3", 200 pounds with a bad knee. I'm riding a 24 speed "Diamond Back Response Sport" (20 inch frame) with "50/50" tires. He a 7 speed "Fuji". We ride paved bike paths away from...