You should know from past riding what you can and can not do in a day. Or how fast. I suspect maybe overstress for a brief period of time. I have knee problems (I am 63, most people my age do) but I no longer stand on the pedals to accelerate from a stop or to power up a hill, I stay seated and gear down instead. That does wonders for my knees.

There is an old theory that pain in front of knee means saddle too low, pain in back of knee and the saddle is too high. I move my saddle about 5 to 8 mm at a time, but adjust frequently when trying to set up a new bike or new seat or something like that. Once I have the height figured out I put a piece of electrical tape on the seatpost about 5 mm above the frame, then if my seatpost is sliding in the frame that 5 mm gap disappears and from a quick glance I know it slipped.

Gearing down for less pressure is teh answer once the pain hits. YOu say you slowed down but that does not tell me if you slowed your cadence while keeping the pressure on the knee or if you geared down and reduced pressure on the knee.

I just got home from a five day trip, first day I tried to keep up with a guy that I know is faster than me. Left knee was hurting at the end of the day. Put a small knee band on my knee at the start of the next day and went slower with less pressure on the knee, by the end of the 2nd day my knee was fine and I stayed at that slower pace on days 3 thru 5. By knee band, I mean a patella (or patellar) support strap, google it for more info. I also carry a better bigger knee brace with me in case I do something stupid. I never sleep with a knee brace or band, that does not seem to help, only use it while up and about.