Photo: mikebaird, Flickr

My knees are shot. Does that mean that running is out of the question? - Robyn Smith, NYC

Running is one of the best calorie burners of all time. It burns between 8 to 20 calories per minute depending on your weight, how fast you're going and your fitness level. Unfortunately, it's also pretty tough on your body. Between 30 and 50 percent of runners are injured every year and up to 50 percent of those injuries are centered around the knees.

So there are a lot of runners out there sidelined by mushy hinge joints wishing they could hear the crunch of leaves beneath their feet and fill their lungs with crisp autumn oxygen. Some will be able to work within the boundaries of bad knees and other joint pain by adjusting the workout. One way to do this is by mixing running and walking intervals together to get just enough of a speed fix and just enough of a break to side-step the aches and pains many runners experience.

For walkers who have always flirted with the idea of running, sprinkling running intervals throughout a walking workout is relatively low-risk and a good way to build up to a full-fledged running session. You may never want to do an entire running workout and in this case, walk-run is a good way to spike up exercise intensity and burn more calories. The amount of running that 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 five- to 10-minute warm-up at an easy walking pace, alternate 30 seconds of running with two minutes of medium- to fast-paced walking. Repeat this cycle five to 10 times for a total of 12.5 to 25 minutes of walk-running. Follow this with a five- to 10-minute cool-down at an easy pace. You can gradually increase your running intervals to two minutes for a total of 20 to 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 five minutes of running with five minutes of medium- to fast-paced walking for a total of three to six walk-run cycles (30 to 60 minutes). Finish up with an easy-paced cool-down.

• If you decide you want the majority 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 five to 10 minutes. Run for three to five minutes and walk for one minute. Repeat 10 to 15 times so that you're walk-running for 40 to 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 up your walking time. Be willing to experiment until you strike exactly the right balance between walking and running.

• If no amount of running suits your body, then consider other methods of getting the same results: Try walking fast -- so fast you have to hold yourself back from breaking into a run. Easier on your knees but can burn the same, and in some cases more, calories than running. You can also try walking up hills. You have to lift your legs up higher than on the flat to move yourself up and forward; the result is a higher calorie burn, plus a killer butt and thigh workout.

My 2 cents: Walking is not a second-class fitness activity by any means, but sometimes you just want more. Walk-run is a good alternative. I'm all for any way that gives you more bang for your fitness buck but has a low risk of sidelining you. I use a lot of walking and running in both my own routines and the routines of my clients. It works. And it keeps things interesting. If you're curious about the number of calories you burn with this type of workout, click to our exercise estimator tool.

Now for your 2 cents: So have you picked up the pace lately? If so, has it made a difference? If you're interested in trying out a unique way to add walking and running to your routine, be the fifth person to sign up to follow me on twitter: twitter@lizzyfit. I'll send you a copy of Chris Freytag's DVD, Walk Yourself Fit. It's a perfect way of getting a workout that's equivalent to a walk-run without leaving your living room. The instruction is excellent!