+1 on brakes and low gearing. Also, you might go with shorter cranks than you normally use, because with shorter cranks your leg is not bent as far when you start putting pressure on the downstroke. Shorter cranks also make it easier to spin a lower gear. If you have knee issues generally (and even if you don't but want to avoid them), pay just as much attention to your bike fit and especially pedal choice and cleat position as you would on any other bike. Take it easy and don't go out riding as hard as you can at first, just see what feels right and gradually work up from there. It's not a fixed gear that's hard on knees - it's grinding too hard when you're too overgeared. So take it easy and start with flat routes and a low gear and see what feels comfortable.