What sling said. You've got a tendon that runs the whole length of the bottom of your foot that's very easy to injure.
I had it pretty bad in the police academy. Do a lot of stretches that involve pulling up on your toe. Also, rolling a soup can back and forth under the arch of your foot helps. And of course, ice it if it's tender/swelled.
Before our PT in the morning, I'd go out in the hallway and lie with my butt against the wall and my legs extended up the wall. Do that for about 10 minutes, and it reduced the swelling enough to run comfortably for the next hour.