Hey Bernard,
I am a few decades shy of 80, but my birthday was yesterday...does that count...?!
Just kidding...
Methinks your plan of a simple out and back is very wise. You will get to see the condition of your return trip (ie. construction, etc.) on the way out.
I would say the most important thing to consider and keep in mind is to "know thyself". Are you able to "power nap" and feel refreshed...? Or do you feel like a banged-up pail of alligator shit when you wake from a nap...? Everybody is different.
If you are able to nap, it might be worth considering grabbing a quick nap at your turnaround point...depending on how much time you have...but if you are doing 100% interstate and don't have to stop too often, you should have several hours "in the bank" when you get to your turnaround.
I would also suggest getting a room as opposed to the IronButt Motel. Yes...it will be an expensive nap, but put it into perspective and consider it cheap "insurance". I completed the IBR last year (for the first time...woo-HOO!) and one of my goals was to get good rest every night (quality over quantity) and it certainly made the experience more enjoyable...
The final thing to remember is that it is
only a ride and the last 100 miles are definitely going to be harder than the first 100 miles. If it isn't going to work this time (because you feel very tired towards the end or overslept or are extremely sore in places you did not think possible or whatever), do NOT try to "push through" or ride beyond your limits...pull the plug and try another day...it will just make for a better story...
I hope everything goes well for you...and if you decide to try the skydiving option instead, my cousin has over 600 jumps to his record and he would love to take you...
Cheers!