I usually find that the more I dread the thing I signed up for, the more glad I am that I did it afterward. If it's really really really tough, I get a great feeling of accomplishment afterward. That isn't to say that it always goes well, but if I'm REALLY dreading it, it is never as bad as I anticipate.
Like the others said, you didn't sign a contract. It's totally up to you (okay, was up to you, since it's 5pm here now). Just remember that you are stronger and more experienced now, and you've already proven that you can do it!
Oh, and I typically regret signing up for every event I do, and wish I hadn't, and I dread everything! But once I get out there, I'm usually glad I did.
So what did you decide? How did it go?? Are you glad (either way)? I'm time-traveling back to this morning, to support you and give you good vibes.