I did the same thing at the ROC meet. ccsteve gave me the advice to start it 1 minute prior, and then hit the 'split' button when you start, so even if you forget to do the split, you have the whole thing tracked, and you can estimate your start from your track.
Steve's advice is good. I do something slightly differently, I usually start mine about 5 seconds before my start time, then don't bother with a split when I actually start, because I figure there's enough else to think about then (remembering to e-punch, then focusing on the map).
The QuickRoute tool is also good for stripping off time before and after an event - When out to Webster I apparently hit the start button again and had a nice GPS route from the park home...-)