I am in the do it slow camp. My gf averages about 8-9 mph including breaks and we just finished an 8 day 500 mile tour. Even a 10 mph average will get you there in plenty of time at the end of the day. Try practice riding consecutive days. First two 20 mile days. Then two 35... etc work your way up. You have plenty of time to get used to riding several days in a row. And go to bed early sleep well dont drink any alcohol or caffeinated beverages later in the day.

I know when I go out on a tour the second day I am usually tired and a little sore then after that I am fine. I think you will be ok.