I feel similar amounts of fatigue after 60 miles fixed vs a geared century.
I rode 50 miles over hills (fixed) yesterday and it's a great workout. The thing to understand here is that it's not a simple mileage comparison - fixed gear requires more torque so it's more of a full body workout as well as the whole gamut of slow and fast twitch muscles. It's a great training tool.
Another reason why it's not such a simple mileage comparison is because a fixed gear helps to 'push itself' - while one can stop pedaling and simply coast on a geared bike, a FG helps pull my legs making each revolution easier then from a dead-stop. For instance, given the same gearing on a fixed gear vs. a single speed (or a geared bike in one gear) I'd much rather climb with the fixed.