Congrats on the progress, that is awesome! It truly is incredible how quickly people can make distance gains on bikes, when a few miles seem like a read toward death to suddenly doing 15, then 25, etc.
Sorry to hear about the pain though. Maybe it was mentioned above or you already have taken care of it, but do you think everything is adjusted properly for you? I guess it would concern me a little bit if you are still experiencing a lot of leg and knee pain after this time, even with frequent riding. Ex. is your seat too high/low, too forward/back. That can make a huge difference with even small adjustments - one knee can hurt like hell if I have my seat just a hair too low. Then as someone said above, making sure you are not grinding too hard in your pedaling and instead doing more easy spinning. If you are on a flat trail that may not be the issue, but thought I'd bring it up anyway. Hopefully a little rest will bring your legs back around. And some good bike shorts for your rear!