I thought I would use today as a rest day, but after I got home I was feeling restless and decided to give myself about 20-30 minutes of core exercise with a minor dose of cardio. My wife has all sorts of gear from her kickboxing workouts so I grabbed her jump rope and 15lb kettle bell and got busy.
Here is the quick workout I came up with. This will work for you if you have 6 minutes or 30 minutes. The shorter the time, the more intense you should make the workout. By more intense I mean... speed up, go faster, don't be a slacker!
Total Time for 1 Round: 6 Minutes
3 minutes - High Knees Jump Rope.
Alternate jumping rope under your left and right feet while running in place. Be sure to keep your knees coming higher than a normal running stride to work the hips and abs. If you don't have a jump rope, just run in place for 3 minutes keeping your knees coming up to waist level (or as close as you can get). Keep good form and posture. This is a good workout on the legs too!
3 Minutes - Alternating: 20 Push-ups, 20 Crunches, 20 Kettle Bell lifts
After jumping rope, drop down and do 20 good form push-ups as quickly as you can. Then do 20 crunches again at a high rate. After you do those, jump back up and complete 20 kettle bell lifts. If you don't have a kettle bell, just do 20 good form squats. If you have any time remaining of the 3 minutes, start back at the push-ups and repeat until the 3 minutes have expired.
Repeat as many rounds as desired. Each round will be 6 minutes so to get a 30 minute core workout you just do this 5 times. That's it. Not too complicated.
I'm going to keep doing this little workout over the next month to see if I get any faster in the pool or on the run. You should try it to!