Do you have a photo of the terrain? As @Yospeedracer said, bike fit is important. saddle height can be relatively easy. Sit on your bike without shoes and have someone support you. Then put the crank arm in the full down position, place heel on pedal and you want a slight bend in your knee. Do both sides since there may be a discrepancy between your legs. Your foot position make also affect your knee pain but I would first get saddle height sorted. Also keep your cadence up to 80rpm. If you can’t do that then you are using to large of gear and knee pain will most likely come.
