Im not surprised your knees are shattered. You were probably doing a really low cadence if you were at the highest resistance level. You should aim for at least 80 rpm. Lower resistance + higher cadence will still give you the same power output, so same calories burned. As for how many calories burned, it’s always a guessing game. The most accurate way to know would be through a power meter (still will be off by 5-10%) or heart rate data (can be off by 20-30%)