My normal watch died the day before the WDW Half this year because I was stupid and wore it into the hot tub. We scoured MGM and found a Mickey Mouse digital sports watch that was waterproof (to 30 meters) and had a stop watch function.
I started the stop watch when we crossed the start line. Since we were shooting for a 15 minute mile pace it was easy to calculate - 15 minutes at mile 1, 30 minutes at mile 2, 45 minutes at mile 3.....
Worked wonderfully! I think there are lots of inexpensive digital sports watches out there with a stop watch function.