I know you've already decided to go (good choice), but me (then 24), DSis (then 31), DMom (then 54) and DAunt (53) all LOVED it. It was DMom's and DAunt's first trip to WDW, and they both listed this as one of their top 3 favorite events for the 5 days we were there. We did not feel out of place at all without having kids with us.
I think we arrived at the park at 5 or so. We were not in costume, but we didn't leave the park until 12 or 1 Am (can't remember), so it was worth it to get those extra couple of hours. I'd get there by 5 for sure. Have a GREAT time - the parade is really neat. You get to see characters that you rarely do otherwise (Cruella, Maleficent, the dogcatchers from 101 Dalmations, the ghosts from Haunted Mansion, Pooh and gang dressed in Halloween costumes), and there's TONS of trick-or-treating. We weren't interested in waiting in long lines, but there were so many lines spread throughout the park that we never waited more than 3-5 minutes. Other adults were getting candy too, so we didn't feel at all strange about it. Toward the end of the evening, CMs were calling out to us asking us to come over and get some candy. The longest we waited was 50 minutes for Space Mountain. HOWEVER --- it was posted as 10 minutes, but we headed there at the time the party ended, and apparently everyone else did also. Every other ride had a wait no longer than 10-15 minutes. The decorations were neat - cool lighting, corn husks around the poles, bales of hay, spooky but not scary music overhead, ghostly Mickey head silhouettes rotating on the pavement outside the entrance, lots of character interaction, the castle lit up in colors not normally shown on other Wishes nights, and the special MNSSHP fireworks that were just great!
We got to ride Splash Mtn and Pirates in our own log/boat. It was wonderful. Reminiscent of the old e-Ride nights (may they rest in peace), and very fun to have that Halloween feel. I really think you'll enjoy it immensely.