Knee pain for newbie fixed riders is most often caused by having the sadlle too high, a lot of fixed fans like to emulate what they perceive as a track position, High saddle and long reach to bars.....in reality when you go to watch six day races the pros will have much more relaxed positions and generally have their saddles lower than they would on a road bike as it allows you to spin easier and is softer on the knees..35 miles on your low gear really isn't that far so I'm guessing your saddle is too high..
Flattening your saddle (if it is pointing down) and pushing it back will actually increase the distance from your pedal to your arse...effectively raising your saddle height...so although sensible suggestions could make it worse....I would do those things and lower the saddle at least 5cm....see how you get on and then raise it in half cm increments 'til you get it just sweet.
Knee pain for newbie fixed riders is most often caused by having the sadlle too high, a lot of fixed fans like to emulate what they perceive as a track position, High saddle and long reach to bars.....in reality when you go to watch six day races the pros will have much more relaxed positions and generally have their saddles lower than they would on a road bike as it allows you to spin easier and is softer on the knees..35 miles on your low gear really isn't that far so I'm guessing your saddle is too high..
Flattening your saddle (if it is pointing down) and pushing it back will actually increase the distance from your pedal to your arse...effectively raising your saddle height...so although sensible suggestions could make it worse....I would do those things and lower the saddle at least 5cm....see how you get on and then raise it in half cm increments 'til you get it just sweet.