Knee Pain

I've started riding a fixed track bike (Fuji Track) on Wednesday. I commute like 15 minutes in the morning and after work home. I realised I have pain in the knees when I try to pedal hard and start getting speed.

I think my posture/frame size/seat height is quite ok. I ride a 48/15 (fuji std.) ratio. Is this too high for a beginner?

Drop the ratio and sort out your position. You can find all the info how to already on the forum.

Pain on the inside of the knee may mean that your kneecap is pulled too much in one direction. See a doctor and get a physio referral if it persists. It's an extremely common symptom.