I found on my first 12hr solo my arms and shoulders suffered, but that mainly came from a lack of long hours in the saddle, just make sure you get out for some 5+hr rides to see what pains you might get and to try and fix it before the big day, also chamois cream is a must, helps prevent serious soreness
All of Rob Dean’s advice on bikemagic and here is really good, he’s the man in the know =)