First centuries and first double on a mid-70's 12-speed steel Mercier, purchased used. About 27 lbs. What a POS. Slow, hard to drive, flexible so that it felt like I was about the tear the bars off when I got out of the saddle. A double on that bike took 16 hours.
After that, many centuries, doubles, long mountain rides, and now brevets on my '99 Trek 5200 carbon OCLV bike (purchased new). Low end Rolf wheels, 23c Vredestein Tricomps pumped to 145 lbs., Terry Fly saddle. 12 hour doubles on this bike. 18.5 lbs. naked. I notice no discomfort from the fast tires and wheels or from the Trek racing geometry. On the contrary, a good handling fast bike is more comfortable. The Vredesteins are durable and have good road feel.
Triple in front 52-42-30, 12-25 in back. Perfect gearing for LD work. I can run the whole cassette from the middle chainring and seldom use the granny or big ring. If I were going to do a mountainous 1200, I'd go to a 12-27.
The lighter your equipment and the faster your wheels and tires, the bigger gears you can turn and the less time you spend in the saddle. That's what I think so far. I'll let you know if I change my mind after I do my first 1200. But I don't think so. This is pretty much what my rando buddies do, except that most of them run Litespeeds. Around here, a rando rider can be defined as someone who would take a $5,000 TdF capable bike and hang a trunk bag, lights, and fenders on it.