How do I prevent neck and back pain, and recover physically/mentally on a multi-day ride?
Don't be afraid to get off the bike.  Take a break every hour or so. Shake out your legs, shake out your arms.  Stetch your muscles.  When on the bike, try to remember to switch hand positions often set a timer on your watch every 10 minutes if you have to to remind you to switch things up a bit.  

Eat before you are hungry, at regular intervals.  Don't wait until you feel hungry to start eating. It's already too late at that point.  

Don't drink too much on the night between the rides. I don't know about the ride you are going on, but a lot of these two day rides have "refreshments" available at the end of the first day. A cold beer can be nice to relax you, but don't overindulge.  You'll regret it the next day.