Butt
I'll put in a vote for butt. I, like you had done very few back to back rides prior to my first tour. Surprisingly, my legs never let me down even though I was going longer and over probably tougher terrain, and doing it for seven straight days. You'll be surprised how your legs respond.
My butt was another story. I very uncomfortable days 3-6, and it wasn't till day 7 (the last day) that I was able to ride in comfort. I had two pair of cycling shorts, one that I knew was comfy for long distance, and a pair that I used for shorter rides, but didn't want to spend the money to upgrade. Day 2 on the latter pair was enough to ruin me for the rest of the trip. The next year I made sure I had two pair of shorts comfortable over longer distances, and really never suffered any discomfort.
Mark