The long plane flights will be good for your haemoglobin levels-almost as good as a LSD day on the bike. Just be sure to eat moderately and try to use the hotel health club's exercise bike for some interval sessions. Don't worry, do the Fondo but I would not ride really long and hard on the weekend before the century. I'd ride the Fondo hard and do 20-30 easy miles after it. On the weekend before, I'd only do 40 miles easy pace simply because nothing you do a week before is going to help and over doing it is more likely. Eat and drink before you are hungry or thirsty on the century. Start the century at an easier pace on fresh legs.
Maintaining fitness while traveling is hard. I used to travel 80-90% of the time. A BikeFriday travel bike helps. You can also rent a bike, just bring stuff to wear aka the kit. Keeping properly hydrated while travelling also helps as does sticking to a sound diet. Even a couple really short but intense workouts while on the road can help maintain fitness. From experience, I would focus myself on maintaining what fitness I had and not losing it.