Echoing [MENTION=158672]FBinNY[/MENTION]'s experience riding FG on the road need not be harmful to the knees over decades of riding if done "properly".
The basics still apply from the old school club riding days when winter base miles were done on FG to build into the big loads of the coming racing season.
Some key elements in long term FG road riding:

Common Sense
Bike Fit
Restricted Gearing
Brake(s) Fitted
Foot Retention
Terrain Selection
Technique
Seat Time
Common Sense

I still ride a FG for distance in rolling terrain long after my 1st season with little change in set-up, technique or mind set.

-Bandera