I would think of it from the other direction? What could it hurt? I don't see anything bad becoming of it; you might want to go home and take a nap afterwards but, so?

I would also recommend having very detailed notes at hand. The last speech I gave (real one before a professional group) I was talking about my own life history and got "lost"/couldn't remember what I was going to say

Do you "like" your speech subject? Try to concentrate on your interest in the subject, and aspects of it rather than, "oh my gosh look at all those people; I'm giving a speech; I'm scared; I can feel myself sweating and my face turning red, etc.".