Sounds like a typical overuse strain injury.

90 miles on gravel at a high speed right out of the gate is a big leap. Not only the distance and speed but the strain one puts on the knees while maintaining balance on gravel. It's different from pavement where you can sort of float the leg and only apply enough power to generate a forward revolution. On gravel your leg muscles are doing lots of micro adjustments.

Try fitting and learning to spin at a higher cadence to reduce stress on the knee and don't take on trips with high mileage right out of the gate if you can help it. If you are going to do that try some equal day rides before the actual trip.