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There are three layers of goals in running a marathon. The first goal is to finish, and if you train smart, you should be able to accomplish that. The second goal is to finish feeling (relatively) good. If you train well and take care of yourself, and pace yourself during the marathon, you should finish on your feet and uninjured. You might be exhausted and wondering, “Has anyone seen a finish line around here?” during the last few miles, but that is different from staggering across the finish line in pain because you ran too fast for the first 10 miles.
The third goal is your finishing time. Many marathoners have a Boston Marathon qualifying time as their goal, and that's fine, but beginners in particular should not fixate on a specific time. And don't fixate on those time charts that project what your marathon time will be based on 5K and 10K times; the marathon is a unique race, and a lot can happen in 26.2 miles. Pick an approximate time that you think you can manage, such as “under 5 hours,” or “under 4 hours” and remember that if you don't meet that goal, your primary goal was to finish.
I'm hoping to finish the half marathon before they close the route down, which is at the 13 min pace....
That's a worthy goal! Good luck, but watch your pace for the first few miles and make sure that you don't start out too fast--ideally you want to run about the same pace for each mile, so you are still strong and steady at the end.