For any athletic event that's seven months out it's best to have multiple goals. There are so many unknowns that it would be crazy to say "If I don't run Time A, I won't be happy."
I have four levels of goals:
Run sub 2:30. My PR is 2:36 - so, this might seem a little beyond what I'm capable of doing. However, I've run 10k and half marathon times that point to a capability of sub 2:30. Still, you might say, "Boston is a hard course. Maybe you should save that goal for a flat course." That's true - but, people have run decent times at Boston, I live in a relatively hilly city (Baltimore) for training, and I have a good training group. Basically, you never know when you'll have a chance to run a good time. Yes, I will need to follow my training to a "T", no injuries allowed, no bad weather on race day etc - but, I know I can do it at some point - why not now?. At least I think I can. Probability it will happen? 5-10% Everything would have to go right - and I would have to run a sub 1:10 half in my tune up race to even attempt it. That being said - most of my training will assume that this is my goal pace.
PR. This is quite a bit easier - but, it's still difficult. I ran a 2:37 last year in NYC and I'm not getting younger (although many runners PR in the marathon into their mid-30's). I'd say there's about a 50-60% chance that I'll get goal B.
Finish without completely blowing up. Although I could get goal B or even A while also blowing up (my definition of blowing up is running at least 2 miles at 30 seconds slower than goal pace) - it's not likely. The marathon is so cruel because it can make you go from feeling the most fit in your life to crippled in a few hours. Leg muscles cramp up, range of motion disappears, basically your body can fall apart until you only march on because you're not sure how else you can get to the finish line. I've run 4 marathons - I've felt like this in 3 of them. So, I give myself a 25% chance of not blowing up.
Finish. Even when I've felt awful I've been able to finish. I've never dropped out of a marathon - and I've never injured myself so badly that I couldn't get to the starting line (knock on wood). I would say that there is at least a 95% chance that I'll be on the start line next April and finish.