First, if you plan to make changes, do it now and not right before the long ride.

There could be advantages to going clipless... requiring pedals and shoes. I don't lift even with clipless, but when I use them they keep my feet properly placed on the pedals, and I feel more relaxed. Some lift on the upstroke, and thus can be more efficient, so there is that too if you do it. I use mountain type shoes, and they also help to keep my feet cooler. I use Crank Brothers Candy pedals when I ride clipless.

Good tires suited to the task make a heck of a difference.

I would make sure the saddle is adjusted properly (for comfort and efficiency), and also consider lowering the handlebar as far as I could while still maintaining comfort by moving spacers or flipping the stem for a better aerodynamic position. You can do this incrementally, moving one spacer to see how it works, etc. A set of bar ends would help you by providing additional hand positions on the long ride. Ergonomic grips could help too.