I just did my longest ride yesterday (about 65~70 miles in 4.5 hours) and I was also a lot more sore than ever before. I had it in the lower back though, someplace that has never before bothered me. And I'm pretty sure it was due to the fact that I didn't sit up straight on the bike much at all. Usually I do a couple minutes an hour of sitting straight up and moving without my hands on the bars.
This time around I also did more hills than usual and I think that made me back tired. And I still get stiff/sore arms after a ride lasting more than two hours. I have to remember to stretch them out every 90 minutes or so.
But 70 miles in a shot is a HUGE difference over three 20 mile rides done during the week.