Interesting. A long time ago, one of my flying buddies said his father flew in distance competitions in the early seventies. The airplanes were very heavy with fuel.

I have a couple thoughts:
If you plan on flying all day, you need to think about ease of flight. I agree that a polyhedral design, although not the ultimate in efficiency, will suit you much better for 8+ hours of flying. You should be able to adjust the trims and let it circle hands-off.

Engine choice is tough. Most model engines are designed for peak power. What you want is good part throttle efficiency. A smaller carb will be effective in this regard. I wouldn't write off two strokes. Some of them are very efficient. Most modern four strokes have very aggressive cam duration and lap. Not so good for part throttle efficiency. Also, propeller choice is critical. I think you want as much pitch as you can stand. That way, when you throttle back, you still have adequate pitch speed.

Your flying technique will be very important. I believe you want to fly at maximum L/D. This is not an easy task. Most sailplanes, for example, fly faster at max L/D than they do at minimum sink. You usually want a moderately quick pace but not fast. If you're near stall, your flying too slow.

Obviously, carry as much fuel as possible and use little or no nitro if burning glow fuel. Gasoline would be the ultimate.

Good luck.