Before I get into the specifics of my knee pain, I'll give you a little background on my cycling past. Basically, I was one of those kids that rode everywhere all the time, even after my license I still rode most places. A little road and mountain. By 19 I had gotten pretty serious about tri's and did maybe 2 dozen a year. By 22 I was racing road and eventually did ok up to cat 3. Never made it to cat 2. I quit racing, riding, anything at 26 for my real love, rock climbing. One and a half years ago I started working as a pedicab or rickshaw rider. Multiple 14 hour days, 5 people up hill, grind it out, sweat sweat sweat. In all that, I've never had any significant knee pain. Even with the running of the tri's. I don't have any knee pain. I'm blessed with good knees.

Until about 3 weeks ago.

I can't seem to finish an evening of work on the rickshaw anymore without one of my knees just killing me.

The pain specifically is a sharp pain at the end of my quad muscle that gets worse and worse and worse as I ride. It's not in the knee cap or joint, it's in the muscle or tendon or ligament at the distal end of my quad muscle on the inside. I generally can grind through because my other leg doesn't hurt so I let it take the load.

Once off the bike, it stays stiff for a day or two, then recovers and feels great. I've done a lot of hiking and climbing lately and it doesn't hurt a bit doing this unless it's sore from a night on the cab.

So here are a few things to ponder.

1. The bike I'm riding is the same one I've had for several months and is the same model as the one I rode for the previous year. I am the only one who rides it, the bike fit hasn't changed at all.

2. I've ridden the cab about 10 days each month Sept - Nov instead of about 15-20. So I haven't worked as much but I'm not exactly being lazy.

3. My right leg (the one hurting) is turned in a little at the knee. Both with clipless and without. I don't know if this has always been the case or if it's something new.

Also, a possible related point or not. It's worth mentioning. When I started, I was pretty much the fastest cab rider in town. I think this had a lot to do with my background. By about the middle of the summer I started slowing down. Now, I'm downright slow. It seems that there's nothing I can do to get my bike moving any faster either. My personal theory is as follows. I don't ride bikes anymore. Ever. I only ride the cab and it's slow moving. In the last year I think I've untrained myself from moving fast to moving slow. The endurance is fine, I could go forever, I just can't go fast anymore. So I'm now trained to go slow. It's annoying the hell out of me and I'll probably jump into a few cat 5 slopfests in the spring to fix this situation. I know that my cadence has slowed down and my slow twitch fibers have hypertrophied and my fast twitch have atrophied. I have to wonder if trying to push at speeds and loads I'm no longer trained for is related.

Any ideas? I'm entering a very busy time of year and can't go around taking days off just because I'm sore.

Sorry, but not taking off days may not be an option with an overuse injury, which it sounds like you have. See if the description of quadriceps tendinosis in this article sounds familiar:
https://www.cptips.com/knee2.htm
I would put myself in the hands of a good sports medicine PT or physician to get a diagnosis and treatment plan asap.

also should look at the fit of your rickshaw bike-- I'm imagining it might not be optimal

There is no question that my bike doesn't quite fit right. However, I've assumed that it was close enough because it's never given me knee pain before.

Now here's a monkey wrench.

Friday night I worked hard and nearly couldn't pedal with the sore leg/knee by the end of the night. Saturday morning I woke up and my knee was stiff and sore. I lounged around all day then went to work that evening, it was sore when I showed up. About 30 minutes into it, all the pain was gone and it never came back. It was perfect. Today I worked another 8 hours on it and didn't have an issue.

Poof, healed, weird.