My two cents: If you are up for extra mileage, I would say Yes, definitely increase your long runs, and for that matter, increase your weekly mileage. But it all depends on your level of fitness and the amount of time that you might have to commit to it. But the fine line is to not "over-train" - and that is a hard thing to recognize.
My personal opinion is the more miles the better, and that endurance (the ability to go the distance) trumps speed - after all, what's the point of running fast if you can't make it to the finish line?
Building up endurance for a specific event is a very individual situation, simply because everyone is different and there is no one-plan-suits-all, thus the somewhat conservative approace that you may be training to. But you have to be careful not to run so many miles that you either wear yourself out or hurt yourself (I've been there and I've done that...). So listen to your body and see if you can squeek out an extra mile or more on your long runs or during the week overall. More importantly, have fun.
-bill