Elementary school kids are accustomed to activities based on short bursts of energy alternating with rest. Introduce them to the idea of pacing by exploring the basic concepts first. Pacing is a tough concept for kids this age so have fun exploring it with them without being concerned that they achieve command of the skills.
These lessons will let kids discover how it feels to run at different speeds, and what their body does to change speed. Kids will also distinguish between constant speed (running at a steady, consistent pace) and variable speed (running at a fluctuating pace).