Here is an email from a reader who is comparing the benefits of machine cardio or hitting the pavement: (my answers are in CAPS) 

 I have a question regarding running on a street or treadmill vs using an elliptical machine.  I find that I can "run" for longer on an elliptical machine vs a treadmill or street running.  I assume this is as a result of the relative lack of impact versus non-elliptical running.  - ELLIPTICALS ARE EASIER THAN RUNNING IF THE ARMS ARE NOT MOVING, BUT IF YOU USE THE FULL BODY ELLIPTICALS (MIMIC CROSS-COUNTRY SKIING) YOU WILL BURN MORE CALORIES THAN WALKING / EVEN RUNNING AT SOME SPEEDS.  BUT I DO KNOW MANY WHO ARE NOT THAT GOOD AT ELLIPTICAL (BOTH HANDS / FEET MACHINES) COMPARED TO RUNNING.  RUNNING IS DIFFERENT AND JUST REQUIRES MORE PRACTICE... 

Am I deluding myself into thinking that by using only an elliptical machine that I am helping myself to be better at cardio and running?  YOUR CARDIO WILL IMPROVE BUT YOUR RUNNING ABILITY IS NOT LIKELY TO IMPROVE THAT MUCH IF YOU ARE NOT RUNNING.  DURING MINOR INJURY OFTEN BIKE, ELLIPTICAL GLIDERS, ROWING MACHINES HELP MAINTAIN RUNNING PERFORMANCE WHILE NOT RUNNING BUT MARKED IMPROVEMENTS ARE NOT LIKELY.  Should I focus more on traditional running techniques?  I WOULD MIX IN SOME RUNNING IF YOU ARE TRYING TO IMPROVE RUNNING SPEED / TIMED RUN EVENTS.  Do you have an opinion on the Elliptical machine?  I LIKE THEM AND BIKE BUT WILL ONLY USE HIGH LEVEL OF RESISTANCE (IE LEVEL 15 OUT OF 20) AS A MINIMUM STANDARD FOR THE WORKOUT

I have personally seen running improvement after suffering a minor injury (shin splint / knee tendonitis) as the non impact of biking actually rested my legs somewhat during what was probably an overuse injury (after running 30-40 miles per week).  After about 3-4 weeks of no running and SPIN classes. elliptical gliders, biking pyramids, and swimming, my run times in a 5km actually got better.

Here are some of my favorite non impact workouts:

Bike - SPIN class - if you have never done one of these, try it at a local gym / fitness club.  You will work harder in a group than by yourself.

Pyramid Workouts - when on ellipticals or stationary bikes, I like to increase the level of resistance by 1-2 levels each minute until I can no longer handle the resistance.  Then repeat in reverse order back to where you started.  This usually takes 20-30 minutes.  The first few minutes of the workout are easy (great warmup), the middle minutes are pushing maximum limits, and the last few minutes are an easy cooldown.  The pyramid makes for a complete workout.

Rowing - If you have access to an ergometer, give a 1500-2000m race a try.  These are hard and will push the aerobic / anaerobic levels depending on your effort and fitness level.

Swimming / Aqua jogging - Swimming laps with or without rest in between is a great way to get a new form of challenging cardio exercise.  I find the rhythmic breathing patterns of swimming translate well into running breathing patterns.  If your knees are injured do not try swimming with fins as it will place too much pressure on knee tendons.  I like to do the following swimming options:

20 x 25m sprints            rest 20 seconds            in between each

10 x 50m sprints            rest 30 seconds            in between each

and

1000m Pyramid:  May need to alter times…

Swim 100m keep pace between 1:20-1:40 for all
Rest 30 secs
Swim 100m
Rest 30 secs
Swim 100m
Rest  30 secs
Swim 100m
Rest 30 secs
Swim 100m
Rest 40 secs
Sprint 100m

Repeat in reverse order but rest with 50m slow swim / breaststroke / back stroke etc…

Remember in order to see the best fat loss in your workout - arrange your workout so you do your PT / weight or fast cardio first, then follow it with a steady conversational paced cardio workout for 20-30 minutes.

In summary, no matter what cardio apparatus you decide to use, just do it!  Something is better than nothing, however, it is very difficult to improve at one cardio event by doing something else.  Typically it does not work that way unless you just need to recover from over doing it.

Also see other cardio options at Cardio Non impact options

If you have a running injury check out www.drpribut.com - this is where I go for reference to all running type injuries.  Dr Pribut is a runner and podiatrist whose website is loaded with helpful tips to reduce pain in running injuries.