A smooth, spinning cadence, 80-100 rpm is more efficient than slogging at the typical lower rates. I am assuming that your knees are as old as you,
, so a quicker cadence is also better for the knees, especially when climbing.
IMO, these are the two important reasons for us "non-racer types" to pay attention to cadence.
Also see:
http://www.bikeforums.net/showthread.php?t=572819
YRPMMV,
Cheers,
Geary