You don't need to break it in. Put it on just before you leave, to maximize the life (and $) of both.
If it's only $20 or so, go ahead and put a new tire on the front, save the old one for your next spare. But a year-old tire with good tread shouldn't give you any problems. The front tire typically lasts twice as long as the rear. Rubber does get "old" but a year isn't much -- unless you store it near electrical motors where ozone can get at it.
-- Mark