I've done backpack centuries on my Pinarello. I dunno if I could roll a double like that. It takes some time to work up to the century, and I could never do that with a cheap or otherwise lame backpack. I do 200 crunches almost every weekday evening during the commercials on Jeopardy.

If you ride upright for 100 miles you will be experiencing some pain also, just in different places.

I ride with a Marmot Zephyr pack with the sternum strap fastened if there's any kinda load in it. I also ride my 8-10 mile commute with at least 10 lbs of junk in the back pack every weekday.

I'd start with some stretching, easy rides with no load and see if it works itself out.

If you really want an upright ride, I like Raleigh Sports 3 speeds. I also like my cruiser and my cruiser-fied MTB.