When I do really long rides, I have one consuming interest - to get my butt off the saddle as soon as possible. I ride a carbon bike, carry little with me, run a saddle that's comfortable for 18 hours, use as aero a posture as I can maintain for a long period, and only stop about every 50 miles.
If you think your carbon bike will hammer your elbows, your fit/position is not OK. I'd work on that first. It's probably the most important thing to focus on as you increase your mileage as it will increase your speed and endurance. In my mid-60s, my fastest double was 10 saddle hours, fastest 400k was 15 hours ET. I'm not a particularly fast rider, just well trained and using 20 y.o. good equipment.
A good goal for fitness is to be able to ride a century on any given day, no prep. A good way to get there is a consistent 150 miles/week or so, with one 60-75 mile ride/week ridden as hard as possible, with 3000'-5000' of gain.
You could try posting about fit over in the Fitting your Bike forum.
OTOH, many riders couldn't care less about their ET long as they finish within the time limit, if any. In which case, your equipment doesn't much matter.
__________________
Results matter