Lost time is the bane of the marathon worker, and time spent in travel is the easiest to lose. If you are alert during your morning commute to work you might consider dictating a report into an audio recorder or making phone calls on a cellular phone. On business trips, always bring something to read while waiting in ticket lines and the like. There seems to be more potential lost time waiting for airplanes than actually traveling on them. Airlines are so cramped, many people will find it more productive to read or write on board rather than to try to use a portable computer.
Portables are great for hotel rooms, especially if the portable has a fax capability.
You must always be ruthlessly honest in monitoring your productivity. The objective is to get as much work done as possible, not to put in as many hours as possible. If your reserves are drained, take a day off and sleep rather than going through the motions of doing work with no accomplishment to show for it. In truth, very few marathon workers will get by with less than one per month, and two per month is a more realistic plan. If you are doing it only because someone else wants you to, get hold of your senses and run for distant hills.