As genixia suggested, I would "test" the existing crack to see if it easily gets larger or not. After that, I would wash the windows, inside and out, and really take a good look at the overall visibility through each. If there is nothing annoying or vision-hindering, then I'd definitely leave it until after the long trip.

When I bought my current vehicle (98 Civic) almost three years ago, I received a nice star-shaped chip on the passenger side of the windshield later that same week. The cause was a pebble spat out by the tire of a trailer truck. I actually spotted the pebble in flight and watched it hit the glass.

Three years later, that star-shaped chip is still there. And it hasn't grown. I would also like to replace my windshield, but I'm not going to do it unless my vision is hindered. I've replaced windows before, and I know it's not cheap.