For me it was being on the bike so fucking much. Not my legs/hips, or the pedelling aspect, but my upperbody and low back.
A bike has always been my main mode of transpo so I think I was well conditioned, but I suppose I naively thought 20-30 miles a day going here and there around town would translate to 80-100 mile days easier than it actually did.
I also admittedly (and stupidly) hadnt ridden much with panniers, and never with front panniers, which I used for that trip. I found myself riding in the drops far more than usual to compensate (Im a 50/50 rider) and got bad tendinitis in my forearm and my back was never comfortable after the first few days. It can be hard to have fun when you hurt. The last few days I was only riding for 5 or so hours.
It was my first 'bike tour'. In retrospect, I would have trained alot more with my whole rig, gotten bigger rear panniers and ditched the front, and started with smaller 40-50 mile days and worked my way up.