If you can pack ride well, then 60 miles at 14mph should be very doable. Until you are sure that you are able to finish, stay in the pack--a large pack actually causes a vaccuum that pulls you along. While you are drafting, keep your front wheel from overlapping the wheel in front of you, try to be smooth and not use the brakes to adjust speed, and keep your head up and your eyes well down the road.
As for your knees, proper pedalling style and gearing will make all of the difference. Stationary bikes are often used for knee rehab.
