Make no mistake: Walking is not a second-class fitness activity. It can get you into top-notch shape, help you lose weight and tone your muscles. However, some walkers want to run at least some of the time. If you feel a need for speed but your joints can't handle the pounding, consider doing a "walk-run."

A walk-run involves sprinkling running intervals throughout your walking workout so you can spike up exercise intensity and burn more calories. Picking up the pace for short intervals makes you work harder without overtaxing your body for long periods of time. It's just another way to get more bang for your fitness buck and add interest to your usual walk.

The amount of running you insert into your program depends on a number of factors:

• If you've never run a step in your life or running for more than a few minutes at a time makes your knees ache, try this: After your 5-to-10-minute warm-up at an easy walking pace, alternate 30 seconds of running with 2 minutes of medium-to-fast paced walking. Repeat this cycle 5-10 times for a total of 12-25 minutes of walk-running. Follow this with a 5-to-10-minute cool-down at an easy pace. You can gradually increase your running intervals to 2 minutes for a total of 20-40 minutes of walk-running.
  
  
• If you can run for at least five continuous minutes without feeling out of breath or achy, try this: After your walking warm-up, alternate 5 minutes of running with 5 minutes of medium-to-fast-paced walking for a total of 3-6 walk-run cycles (30-60 minutes). Finish up with an easy-paced cool-down.
  
  
• If you want most of your workout to be spent running but still want to give your joints a break: Start out with an easy walking warm-up for 5-10 minutes. Run for 3-5 minutes and walk for 1 minute. Repeat 10-15 times so that you're walk-running for 40-60 minutes. Finish up with an easy cool-down. If your knees feel sore after this type of workout, you can always back off your running intervals and increase your walking time. Be willing to experiment until you strike exactly the right balance between walking and running.