I just get in the drops and keep grinding away.

One aspect of long-distance riding that I hadn't figured on was that with longer rides, you can also get stuck riding into the wind for longer times. On my 1200k this spring, I rode solo into a headwind for about the first 115 miles. On one of my 300k's, I had almost a 100 miles of headwind coming back in. There's no magic solution, just keep riding and work your way in.

(Oh, draft a tandem if possible, but not always feasible!)

One other thing, if it's an out-and-back as you say, just ride out, if you get too pooped too early, just turn around early, no law that says you have to go the whole way just because you start out.