Most of my routes are hilly without being stupidly steep but I find little real difference between my fixed and geared bikes. My "good" fixed, a 531 framed Carlton has a 69" gear but is probably as high as I can go in this area.
For this winter, on the advice of an old club cyclist I've got to know well, I built an old gas pipe special with a 62" gear for winter training and it's insane on the downhills but somehow I find it brilliant fun and I can now spin well enough to almost match my times on the Carlton even though the bike is considerably heavier and my top speed is lower due to the gear ratio. I must be making up times on the climbing. If you really want to get fit, I think you need a low geared fixed.
For this winter, on the advice of an old club cyclist I've got to know well, I built an old gas pipe special with a 62" gear for winter training and it's insane on the downhills but somehow I find it brilliant fun and I can now spin well enough to almost match my times on the Carlton even though the bike is considerably heavier and my top speed is lower due to the gear ratio. I must be making up times on the climbing. If you really want to get fit, I think you need a low geared fixed.