Rule #1 - Always start your GPS when you get out of the car to avoid forgetting to start it at the start line. You can always trim off the excess afterwards, but it hard to create route data that you missed recording.
If you make it a habit to start it out of the car, then you can make it a habit to start it before you punch the start brick :P
I somehow even managed to turn off auto pause and set up my map in the one minute before the middle distance start.
I'm just amazed you stopped it as long after your race finished as it took you to start it after the start, thus TBT has picked it up as your legitimate trace (not so for the middle distance, which varies by 4 minutes).