I am going to file a minority report.
Fist time backpacker? New equipment? No idea of terrain? 18 mile first day?
I think this is too much. Yeah, you could probably live through it. Is that what you want?
Just to give you an idea, I am double your age, but also in great shape, cycling 20-30 miles a day, and riding hard. I don't ever plan a trip that includes an 18 mile day. Why? Because I hike at about 2 mph over reasonable terrain with a pack. ANd by the time you add in a couple of hours for breaks, lunch, snacks, I gott tie my boots again, wait, I have to pee....that's gonna be more like 11 hours on the trail.
And I have to ask what kind of people would invite a newbie on a hike like this?
YOu are better off starting with something shorter that gives you a few options in case you are tired, blistered, or just sick of hiking.
Just my 2 cents