For safety reasons, I decided to plan the trip to avoid any riding after dark.  In addition to the crepuscular critters that abound, the tire treads, trash and other road hazards that you can't see in the dark have caused enough near misses in the past to produce a healthy respect for having daylight to ride by.  Committing to daylight riding only in June is no big deal, but in November with 1,300 miles  to cover in 2 days with limited daylight hours this new benefit became apparent.

The Garmin III provides a continuous readout of precise sunrise and sunset time not only for your current location but for all waypoints that you program in. As you traverse latitudes drastically during this season the day length shifts not only with date but also with latitude and so your time of darkness onset is 40 minutes earlier in El Paso than Sacramento in November. Using the GPS provided sunrise/sunset data, and adding 20 minutes each side for twilight let me plan my daylight travel quite precisely. 

There is a period called "Civil Twilight" that precedes sunrise and lags sunset. That is the period when the sun is below the horizon, but it isn't fully dark yet. It varies, but 20-30 minutes or so works as a reasonable plan. So to maximize my riding time, I set the motel alarms for 1 hour prior to official sunrise, giving myself 30 minutes to get in the saddle, and then be launching out when it was just starting to lighten in the Eastern sky. The night arrivals, were timed to be able to find lodging while still having some visibility.

There is a feature in the GPS that gives you an estimate of your arrival time (ETA) at each of your waypoints in advance along your route (using the "Routes" function of the GPS III), and on the same line gives you Sunset at that location. These are updated as you go by the GPS based upon your present position, average and current speed. So as you watch it roll along, you get a pretty decent idea of how you are doing, and where you are going to need to call a halt and find lodging. The question of whether or not you can make the next town before dark gets answered pretty precisely.

Of course the GPS doesn't know how curvy the road is in front of you, so you need to approximately match the road profile with waypoints. On Interstate highways this is relatively easy by making a waypoint at each major direction change, or significant interchange ("significant" = a place you are doing something such as exiting, stopping for gas/food, etc.).

On secondary roads, particularly those that have a lot of twists and curves, you can't do much to solve the problem other than remember the GPS is calculating future distances as a straight line between your current position and the next waypoint. And if there is a lake in the middle well that's your problem. Of course by inserting multiple waypoints so that with a series of straight lines you have drawn an arc around the lake, that solves the problem.

The sunrise sunset feature was useful and helped me manage my safety envelope.   Also, unless locked in twisties, the GPS gives you a very accurate readout of your speed, so you can compare that with your speedo and have another margin of safety with regard to the friendly bears on the road.  When my route plans changed en route, a night a few minutes of reprogramming would reset the route for the next day.