How much rest is required between cardio workouts?

I'd also like to know this.

I alternate days running and cycling. Both give me a good cardio workout, but running uses the hamstrings while cycling uses the quads. So I cardio train six day a week, but rest one muscle group each day while training the other. The two exercises complement each other beautifully. I strength train three days a week with three or four runs and three or four rides (not four of both). Two-a-days are doable as long as you allow time for each muscle group to rest. And I take a full day off once a week.

As many have mentioned in the previous posts, you can alternate between the types of cardio. However, if your cardio is mostly with running then alternate between the intensity level don't follow a speed/interval training day by Hill training days etc. Include easy runs, tempo run and split runs in you weekly routine.

Remember there is no such thing as being too slow in running better to err on the side of caution than to get injured. More does not necessarily mean better. I know you are a man but in this month's issue of Womens Running Magazine, there a very comprehensive 1/2 marathon training plan. It actually breaks everything down to the intensity level of each run. Check it out at the library or barnes and nobles. Good luck.

I wanted to add an additional thought. Yes, one can wisely alternate muscle groups and do cardio on back to back days, but, taking one full rest day a week is still helpful. Some serious studies have shown that exercise is just another type of stress to one's physiology and health is enhanced by periodic full rest. At one level our bodies don't care if stress is exercise, divorce, job loss, or whatever. It needs rest from stress whatever the source. Maybe we can't control emotional stressors, and exercise does help there, but my point is that resting one muscle group while exercising another works but it is still stress to our physiology and we need rest from all exercise periodically. I am doing two-a-days (sometimes three) and I am exhausted by Sunday (my rest day.) I am seriously considering adding a second mid-week rest day if I am going to continue with intense two-a-days. My body is asking for rest after three days, but honestly I don't know if I can give up two days a week to rest. I love my trail running, road biking, and P90X and now on top of my program I am hill walking with my wife to encourage her to exercise. Anyway, rest is crucial! That's when our bodies grow muscle and recover.

Having done an hour of circuits last night, and having gone for a 5.5k run this morning, I can safely say 14 hours is NOT ENOUGH TIME. My legs are going to fall off.

I was doing cardio everyday and started to get breathless at night, so went to my doc.
Apparently, we need a rest day every 4th day. It lets the muscles relax and heal up.

Just a rest from cardio, mind, not a nothing day.

So maybe every 4th day, just do some toning/weights and then take your Sunday to chill and do nothing.

When I read your comment in the AM I was resistant to your suggertion but now after completing my two-a-days (actually three today) I have to seriously consider more rest. I am off work Friday and Saturday so have the opportunity for some serious exercise time, but my body is so tired that I can't take full advantage. I wish I had done only one exercise today so I would be fresh tommorrow. Oh well, live and learn. I am rethinking my schedule based on my experience and your advice. Thanks.