Select another "mission" before race day. Even with the best preparation, there will be a natural motivational lull after the race. When you've spent months working towards a specific event and you've reached the finish line of a significant physical test, even the most focused athletes experience a psychological letdown.
Select another "mission" before the big day. Write the date of your next project on the calendar, journal, etc. The farther ahead of your first goal, the better. It's best to shift gears in selecting a different type of mission: a scenic trail, a weekend trip to a big festival event, a group run or walk with friends you haven't seen in a while, etc. If you wait until after your first "mission day" to chose another goal, your letdown will be more severe.