It's really whatever you feel comfortable with. I usually plan on a 10 mph average pace so 60 miles would take me 6 hours in the saddle. More miles mean more miles in the saddle. Where I live, there's about 15 hrs of daylight this time of year. 60 miles of riding gives me 9 hrs of not riding. That's quite a bit of time to do whatever. A swim break can be awesome if you're riding along a nice river or a swimable lake.

My big shock on my first tour was butt soreness. 6+ hrs a day on a bicycle seat will be noticed. I have to train my rear as much as anything. Similarly, you need to be aware of the possibility of getting saddle sores - not fun! Keep clean and change often. Bring along some Chamois Butt'r, Vaseline, or something similar just in case. If you stay clean and dry, you shouldn't need it.

Training ahead of time can make a tour more enjoyable. If you're riding with others, it can be the difference between having a good time and being miserable. (If they've been training and you're struggling to keep up). If you are riding with others, try to figure out what their idea of a tour is and how they'd respond if they needed to scale back their distance or their pace. If you're riding alone, don't sweat it. Just ride as far as you want, however you want!