Thanks for the Garmin advice and the encouragement from everyone! I figured out how to reset the timer -- Junebug was right about holding down the "lap/reset" button. I was just pushing it earlier, and it reset the lap but wasn't re-starting the timer, but holding it down was the right move.
I'm still a little sore from Sunday's 18, but I plan on running this evening after I get home. I'm gonna need the stress relief today. It's nearing the end of the semester for me, and the rush of students and their needs is exhausting. I've been like a machine today: reading essays, meeting with students, answering emails and phone calls, and I can't wait to get home, tie up my shoes and free myself from the stress that the end of a term brings with it.
Not to mention the holiday stuff to get done (gift buying/wrapping, writing cards, planning trips); tis the season to be busy!
P.S. It's funny how this marathon has educated so many in my life about running, not just myself. Today, my mom wrote me an email to let me know that she'd been talking to a runner friend who told her about the "wall" at miles 18-20. She wanted me to know about it -- just in case I hadn't heard of it :)