Cadence smadence, this old debate's being rehashed again.
Back in the olden days, like 100 years ago, there was only 1 gear on a bike, usually fixed. Euro racers varied their cadence from 20-180 RPM depending on incline, road conditions and wind speed. They could travel over 200 miles on a 40 lbs fixie.
For 30+ years the standard gearing on a racing bike was cranks 42/52, freewheel 13 or 14-24. Cyclists had to get off their rear ends and pump themselves up hills at low RPM and spin like crazy downhill or in a sprint. These guys mastered a very wide range of cadences and had more efficient cardiovascular systems and nicer muscular bodies, much more so than cyclists today.
Bernard Hinault, the 2nd greatest cyclist in history had this to say about perfect cadence, "Do both low and high cadence training, low cadence is good, it builds strength."
Now would you rather have the legs of Bernard Hinault or Andy Schleck?
All this emphasis on high cadence pedalling only is based on artificially hyperactive EPO bloodstreams of pro riders where cardio trumps muscle. Normal non dopers can't pedal efficiently like Lance Armstrong at 110 rpm. I say mash away until your knees say otherwise. Do spin too, for heart strength.
Just listen to your body, mash when you can't breath, spin when your legs are thrashed.
Go back and forth and you'll be a better all round cyclist and get the buff legs.
Spin only, and you'll be skinny.
When you get old with creaky arthritic knees never dip below 80 RPM on climbs, 90 RPM on flats.
Ride a single speed in 10 mph+ headwinds if you wanna force the issue from both extremes and get a great leg/heart workout.
Stop fixating on the cadence thing. Some serious mashing is good for you at regular intervals. Sprint intervals are even better, especially if you don't get your minimum mileage in.