How far, away, mileage wise/time wise? 200 miles, 300 miles, more? My first long day was over 350 miles. From Baltimore, to a picnic in lower central VA and home again all on the same day. The wind kicked up big time on our way home, to add interest.
What is the greatest distance you've ridden so far? And how did you feel? Any discomfort (seat, back, etc)? Stuff to consider adressing.
A good rule of thumb, to keep creeping fatigue at bay, is to get off the bike, take off the helmet, and walk around for 5-15 minutes, about every 60 to 90 minutes of riding. (Since gas stations can be so depressing, I'll make gas stops "short and quick," and plan to stop/refresh at the next rest area.
Since the first thing to go as you become fatigued is judgement, it is really important to not push yourself too hard while riding.
(1) How to prepare physically is my main question? What are some guidelines for the frequency of taking breaks? How do my husband and I keep an objective eye on making sure the other is still capable of continuing on or turning back to home base?
Why turn back? Why not take a hotel partway there, rather than bail out entirely? Verizon Wireless VZ Navigator can find you a hotel, if you don't have a GPS (surely the other wireless networks have some similar feature, if you don't have Verizon).
If you've got a looong ride, eat a good breakfast, start early and plan to take a fairly long sit down lunch mid-day. You'll be better able to continue the rest of the distance.
P