Training Plan

JohnnyKay definitely has it right. Stretching the plan because of anticipated missed workouts defeats the point of the periodization. You can add to the beginning of the plan because the race is further away than the plan calls for and you want to fill the gap but start training right away. You could also add to the beginning because you're not quite yet at the fitness level that the start of the plan assumes.

Missing a workout or two a week isn't the end of the world. If you plan ahead and are disciplined, you can work around travel issues in most cases. Pretty much every hotel has a treadmill. Most have a stationary bike which, while not ideal, is at least a workable substitute. For swimming, go to swimmersguide.com. They've got a massive database of swimming pools that you allows you to find a place to swim in just about any city you're in. I trained for a 1/2 IM while traveling every week.

As for injury, that's a whole other issue. If you're getting injured consistently while training, you need to figure out why. Overtraining has nothing to do with how many weeks in a row you train. It's due to training too many weeks in a row at a level your body isn't accustomed to. I'd wager that most pros train year round at levels that would leave us broken on the floor. They've got the base to do it, though. They kick it up to an even higher level when they start peaking for a race.