Now is the time for most of us to take our cardio outside, and what better way to work on your leg development than with a track workout.
I do different track workouts every time I go to the track. The possibilities are endless.
Here's one workout that I've done a few times, or something similar to it.
This is based on a 400 m track, but it can be modified for whatever size. Some indoor tracks are much smaller, esp in gyms.
---2 laps jogging warmup followed by a light stretch
---SPRINTS around the track - 100 m sprint, 100 m light jog/walk, 100 m sprint, 100 m light jog/walk. Repeat 2-4 times. Just make sure that the sprints are on the straight part of the track...not a good idea to sprint around the turns, as you can strain your knees.
---one lap light jog/walk to bring the HR back down
---SKIPPING - Skip as much as you can around the track. Walk as needed to recover (you will feel the burn!), but be sure to pick it up and skip again. There should be more skipping than walking. 2-4 laps.
This is great for the calves!
---Super Sprints - 50 m as fast as you can. Turn around and walk back. Repeat with no additional rest. Do 6-10 of them.
Do 6-10 more backwards...run 50 m backwards as fast as you can and then walk forwards back to starting place. Repeat with no additional rest.
---2 laps jog/walk to cool down.
Depending on your conditioning, how much you run/walk, and how many laps/repetitions, this should take you somewhere between 40 mins - one hour.
Stairs/bleachers can be added at the beginning or end, depending on how badly you want to hurt. 
Here it is for me once per week:
15min Dynamic warm-up (too much to list but usually 20 yards)
400 meters, sprint straights walk curves
Run bleachers on seats , sprint everyother section, box jump everyother section (our bleacher have 4 sections) 3 min rest between each run.
Sled pull with 45 to 90lbs (depending on how I feel, I'm on low carbs right now) 40 yards backwards, 40 yards pushing, 5 min rest between each run.
keg toss - 5 sets 5 tosses
400 meter cool down
Stretch
go home pass out!