Beginners: DAY OFF.
Intermediates: Hill repeats x 8. Find a hill that takes 45-60 seconds to run up. Run up as fast as you can. Walk down. Repeat until you have completed the 8 reps.
Advanced: Sprint Intervals (4x800 meter runs; 4x400 meter runs) Beginning with the 800s (about 1/2 a mile), run as fast as you can, then jog for 2 minutes. Repeat until you have completed the 4 800s. Complete the 400 meter runs in a similar fashion. However, jog for 60 seconds instead of 2 minutes.
Carl Lewis / Courtesy Eric Cressey