I would be careful with the hills until you get the hang of fixed. It's the downhill that is a bigger issue. This is one place where some sort of foot retention is really helpful. Keep in mind your cranks keep moving when you are going downhill so your legs have to keep moving and most likely at a much higher cadence then you are used to. If you have a momentary lapse in concentration you feet could get bucked off the pedals and then you are in trouble (hence the foot retention).