Longer rides do highlight the importance of having your bike set up correctly. I really need to make some adjustments to my fixed gear bike because I was starting to find it very uncomfortable towards the end of the 300, and in particular my knees were suffering badly.
Flat rides on fixed mean you don't get out of the saddle that much. The first and last 300km of LEL'09 (which had very flat start/finishes) I had similar problems. I've also had similar discomfort on the Fairies 300 on fixed but no discomfort on the BCM 600 (where I got to ride out of the saddle quite a bit). It's harder (but not impossible) to get out of the saddle and stretch/stomp on fixed.
On an undulating (but not stupidly hilly) route I don't find fixed much different in terms of speed from a geared bike. They're both roughly the same in terms of mechanical efficiency so putting in a certain input power will get you roughly the same speed on both; what varies is the cadences that you are putting that power in at.
If you don't have lots of fixed miles in your legs then you'll find the variation in cadences will tire you out much earlier. Being able to put (say) 200W in for a period of time at 90rpm is one thing, being able to put something near that in at 25rpm for a prolonged period is quite a different story.
Long/steep descents are obviously much easier on gears as you're not having to spin away (even not putting any power in requires quite a bit of work for the legs) but generally I find I'm less lazy on fixed as there's nowhere to hide. I can't bottom out to a 30x29 gear (or similar) and spin up at 6kph, on fixed I tend to stall when climbing at anything under 25rpm and so generally climb a bit faster on fixed than gears as it means my lowest speed is ~8kph. Most things over 15% have me dismounting and walking (although this is often the same on the geared bike).
Steep climbing on fixed is essentially stair climbing, it's just your body weight on the pedal forcing it downwards and then the bulk of the work is moving your CoG back up again by standing on the other (now higher) pedal. A bit steeper and you're also using the upper body workout of wrenching the bars. Steeper still and you add in pulling up on the lower pedal too. (OK, a bit of a simplification, it's not so simple a 3-stage process as this.)