I haven't read through all the posts so I may be repeating what somebody else has already said, sorry.
I had a terrible time keeping an old Yamaha in tune but I was hauling to and from the boat (don't like to keep too much stuff on board, it tends to grow). Got tired of ever tuning so I started leaving it and once it acclimated to the humidity it was fine. Of course, the boat is on the west coast and humidity isn't as high as say, the SE. You should be fine leaving an instrument aboard in CO.
As for feeling sluggish on board. Moving around on moving boats will make you hurt in places you didn't even know you had at first but you too will acclimatize to being on board. I used to sail with a guy who was in his middle ages, was way bigger around than he was tall but could move around on an Olson 30 like a monkey. He spent every weekend sailing and put us younger guys to shame.
I wish you luck with your fitness program.