For most people when they think of vacation they think of "relaxation" or "rejuvenation" as it is a time to get away and reset. However, my approach to relaxation is to literally rejuvenate my training, both physically and mentally, and there is no better time to do this than taking a hiatus or vacation.
When you stick to a marathon training plan you usually stick to the same routes as the distances and surroundings are familiar. Some enjoy hitting the tracks, others prefer the roads of neighborhoods. Regardless, they have the same things in common: consistency. And for runners this is a good thing to have...until you run into the training rut.
Just like when you get into a mental block at work and you have to step away and change the environment you are working in to continue on your project or in strength training when you want to "confuse" your muscles by switching up your method, the same concept applies to running. I have found that by changing the natural environment you are in, altering your mindset, and putting yourself in a relaxed state can help reset your body.
Most research suggests that it takes close to two weeks of repetition to set something as a habit. So if you are lucky enough to take close to a two week vacation you should. At the end of it you could significantly increase your chances of setting up some positive habits, not just in training but also in your lifestyle.
As for me, I try to take close to two weeks off per year in one go, always around the middle of the year. For some it may be two weeks in the winter, others may prefer a week in the spring and a week in the fall. Whichever is your preference, you ought to take that time to get away and have the ability to reset. As much as I would enjoy running non-stop in DC, I surely wouldn't mind taking runs along the beach or some unknown trails.
So go out and run somewhere new and tackle your training with a newfound vigor.