Good points JD, and hi again Sara.
The races will give you intermediate goals, so you can pat yourself on the back as you achieve them, rather than just having one big goal right at the end. This should help with motivation. Okay - you could do the same with target distances, but a race makes it seem a bit more serious, plus it will help you get an idea of how you're doing, plus help you get used to running as part of a crowd.
I'm not sure whereabouts you're based, so I can't suggest any races, but distance wise maybe a ten miler (probably fairly soon if you can do 6.5 okay, but maybe do a couple of eights or so before trying it). Half marathons are common in the spring, which is a good barrier - you're half way there and if you're like me you'll start thinking of the distance left as it comes down, rather than counting up! I also started thinking "I can't possibly run the same distance again", but trust me, your body gets used to it. As a final race, maybe an 18 or 20, but not too close to April 13th as you need time to recover. At least three (four is better) weeks beforehand. Worthing has a twenty mile race if you're in the south of England that is around the right time, and I think there's one at Southend too.
Agree about the visualisation too - remember your final goal, and keep reminding yourself about it, and thinking what it will feel like. It's an awesome experience runing it, and finishing was one of the most amazing moments of my life.
All the best, Iain