For us slightly older guys, going CLIPPED IN, has saved my knees, even when riding a butt killing DF MTB, as my cadence went from 50-60 up to 80-90 RPM!!!!
With each leg weighing approximately 10% of your total weight, so 7 to 12 kg weight which would require MINIMUM of 7 to 12 kg load on BOTH pedals at all times(360 degrees) to keep your feet on the pedals.
Pedaling at 80 rpm with a 7 kg load on each pedal for 180 degrees, produces 100 watts of power.
So for a 70 kg rider, the power required to keep your feet on the pedals, without cleats, is 200 watts!!! so to create 100 watts output, 300 w input is required!!!
I did not realize how hard it was to pedal without cleats, after 10 years of using them, until I rode 6 km to work one day requiring MANY stops in pedalling to give my legs a rest!