There's no need to consider Marcy a "backpack" hike unless you're trying to go out of your way to camp out. Yes, it's about 15 miles, but most people can hike it in 10-11 hours. Check the weather report before you leave and you can leave most of the gear suggested here at the motel or wherever you stay. You'll save your knees and make the hike more pleasant the less weight you carry. We met a young fellow who never hiked anywhere without carrying a tent, sleeping bag, extra food, etcetera, and needed a knee operation at about the age of 20. I couldn't bear my pack on my back anymore so I switched to a hip bag in 2000 and I'm much happier.
Animals. I'm 22 High Peaks into my second round of the Forty-Six and I've never encountered a bear while on a hike. Saw one way down the road while driving through Algonquin Provincial Park in 1990, and at Clark's Trading Post in Lincoln, New Hampshire, when I was a kid. We met a deer once in the Adirondacks, just after leaving Johns Brook Lodge on our way to Yard Mountain. That was in 1999. Nothing else wild bigger than a woodchuck, and the woodchuck was near the St. Huberts parking lot just last week when we climbed Noonmark. I've seen plenty of deer tracks, but only the one deer. Bear track once, near the Flowed Lands. Never a moose track. Thought we saw a moose poop last week between Moose and McKenzie Mountains, but, as it was only one doot (as my ex-wife would have said), it was probably from something else.