Bostontrevor has it right.
Most knee pain comes not from the degree of exertion, but from the mechanics of exertion.

We all have different bodies, but as general rule, we all learned to ride a bike by mashing on the pedals.
This puts the big burden on the muscles in the front of the thighs, which not only straighten the leg, the also pull the lower leg up into the knee joint, increasing pressure on the knee.
If we can transfer some or more of the work to the muscles in the hips and buttocks, then we don't pull the lower leg up into the knee joint with such severity.

We also have other mechanical issues, mostly having to do with the typical American habit of walking around with our toes slightly out.
Not everyone does this, but most Americans do, and this creates its own set of problems when pedaling.

In any event, to wake up the hips, first stand with feet about as far apart as when pedaling, and with the toes straight ahead.
Put the right hand on the right hip and buttock.
Step backwards with the right foot, with the intent of keeping the toes perfectly straight ahead.
For most Americans, this will require them to consciously turn the right heel outwards in order to keep the right foot straight as it steps backwards.
As one steps backwards, turning the right heel out in order to keep the right foot straight, he will feel some muscles working under the hand on the hip and buttock.
Put both hands on both hips and buttocks and slowly walk backwards, turning the heels outward so that the feet stay straight.
If the rider can awaken those same muscles while pedaling, in most cases, the knee pain will go away.
It won't hurt to get a cog with more teeth, as a higher spin rate and an easier spin allow a person to concentrate on learning instead of powering.
Also, imagine a rubber band between the heels and keep the heels apart.
Screw the toes inward on the downstroke and the heels outward, not so much that one can see, but so that one can imagine it.

Pigeon-toed people have other issues, and should probably see a physical therapist or foot doctor.