Lane, when you say you are starting to notice how it feels to get older, I assume that means the typical stuff of we begin to slow down a bit, maybe have a few more aches pains when we lift 60 lbs. or bend over to pick something up, etc. If I am understanding you correctly, then maybe concentrate on those places that might be a bit more physically challenging right now...those places to which you may not be able to someday opt for a leisurely, easy river cruise to see.
You have traveled a lot...wow!! And you got an early start compared to so many.
I agree with all the previous posters...you've received some great suggestions. All I would add is to make a list of those places (in priority order) that IF two years from now you found out that you could never travel again, which places would you really regret never seeing? Or are there places that you think could change radically (politically) that you should go now while the coast it clear (so to speak)? Have you visited every continent? If not, maybe concentrate on a place or two on a continent you have not yet visited?
When we put a high priority on travel, I made a Tier One bucket list. Now that we have accomplished that list, I'm somewhat like you, the travel brochures/catalogs fill my mailbox with so many wonderful trips, so many places to see and do. I really want to do them all......sort of like how when the Sears and Roebuck Christmas Catalog used to arrive when I was a kid, and we would start dog-earring pages...but then had to get down to our two (modestly priced) items to put on the Santa list. Travel is often the same way.
But, do the more challenging trips now...those that might require more climbing, walking, longer flights, etc. Those will only get harder as we continue to age. Or, on the other end of the spectrum, decide you are going to take a year off from travel. Nothing will help you to decide more quickly where to go, than deciding after you have decided not to go somewhere (I hope that makes sense).....a trip will FIND YOU!!!
At the end of each year, I do what I call "travel flirting." I flirt with lots of ideas, then one or two of the trips find me....either a very good offer comes in, and I pull the trigger on the trip by booking. Or I read about a place, and then an offer comes in......and, as one of the above posters mentioned, that is usually combined with finding a great deal on airfare. It drives my spouse crazy when I start bouncing lots of ideas off him, but he knows that's part of the process. If he seems more interested in one place than another, that factors into the decision.
You are not alone. I think many of us find ourselves exactly with the same feelings you have.
But, you are so very blessed to have made priority a travel in your life and to be able to continue traveling.
Oh, the memories you have (and will continue to make)!!!!