FIFY...
If the issue is the knee, why would going anywhere else be the 'solution' /remedy ?
"but every time it starts feeling better I overdo it slightly and take one step back after two steps forward"
a resolution for an eye irritation is not a poke in the eye...
better to determine what is causing the problem,
your seat placement exacerbating the knee?
foot on the pedal, rotation issue?
too big a gear for an old farte?
sudden injury which is now exacerabated by stress of hard pedal strokes ? - they happen...
cardio is not an answer for mechanical injury...

working on lighter gears and smoother pedal stroke often help to resolve the injury, by reducing the causative stress, and also makes it less likely to cuase further injuries down the road.
side effect is more efficient pedaling and opportunity to build more 'power' down the road.
This happens to be a good time to do that in the No. Hemispshere....
... is this thread about something else...?
Ride On
Yuri