200 miles- but trouble

Rode 200 miles yesterday at 16.5 mph- and the legs felt great. However, the left knee did not, with much pain in the last 30 miles- just no power on the hills- and thats where it really hurt. Will go for an easy ride today (Saturday) of 15-20 and see how it feels. The goal is to ride 100 on Sunday- but thats all determined by how the knee feels.