I'm an ultra runner, so I'm probably not a good one to answer compared to most people here. :-) But I think a different perspective can be helpful...

First, I'm not chronic cardio. I run marathons on as little as 15 miles per week average, and utlra's up to 40-42 miles on less than 30. I'm shooting for 50 in the fall and probably won't average more than 30 miles per week leading up to that.

I mix in other work like crossfit, movnat, lots of hiking, a little mtn biking, some paddling, traditional weights, etc., to make it all happen.

I consider running part of my "play time" as outlined by Sisson. Is that an excuse, or is that cheating? I don't know, but I love to run, so it works for me...

I do think that 6 days of cardio with HRM at 80%++ of Max is not so good for you. On the other hand, I think training at your maximum aerobic rate, is not as bad for you as the chronic cardio camp says it is. Much of my running is at or just below my max aerobic. But I will mix in anaerobic work in the form of races and intervals, but only for a small period of time.

For many people, runners included, max aerobic rate is going to feel awfully slow when 1st attempted. My max is around 135-145 bpm, which is a 9:30 - 10:30 pace depending on how far into the run I am. However, my 5k race pace is just above 6:30 and marathon pace just under 8:00.

I run mostly by feel, but will use an HRM every now and then to ensure my "feel" has not drifted from reality.

5ks are awesome and a lot of fun. It's usually about 30/40 minutes of jogging for me and my girlfriends and we spend the time chatting and catching up with each other. Obviously, we aren't very competitive about it, we are there for the free shirt and the company!

I am pretty comfortable with 1/2 marathons also. I've done 4 of them and can do them without a lot of training. Sprint triathlons are also a lot of fun, though I have only done pool swims and not open swims.

It's my opinion that it's the training that makes for chronic cardio. The first couple 1/2 marathons I did, I religiously stuck to the training schedule I was given. In the end, for the actual event, my body was pretty beat up from the constant running schedule. More recently, I've learned to rely on my regular exercise schedule and level of fitness to get me through the event. My 5k times this year have been better than any others!

I ran from Portland, Oregon to Keene, New Hampshire in the summer of 1980. That's about as far as I'll ever want to run.

i run 100 miler ultra marathons about once a month, and 50 miler's almost every other weekend there isnt a 100 mile race. I use the 50 miler's pretty much as my long runs now, and crossfit as my cardio/endurance training during the week. Are you currently running? if not, start out easy, and increase your runs by no more than 10% each week. Take it easy, running should be enjoyable! One thing to be aware of, is your form. Make sure you have good posture and are not slugging and bent over (i see so many runners do this, and their face makes them look like they are in pain...running shouldnt be painful)

My personal limit is a 5k. I am doing (almost done!) the Couch Potato to 5K program and I'm looking forward to my first race.

I think ~3 miles up to 3x/week, for me, is low enough to avoid most disadvantages of chronic cardio, but at the same time high enough to reap the benefits...perhaps the physical benefits are minor compared to other forms of exercise, but I find the mental benefits to personally be worth it.

One of the races I'm looking forward to is a Warrior Race. It's a 5k that has obstacles along the way. I think this is a great way to let my paleo/primal self out. 8)

my personal running limit is several sets of all-out sprints.

Since turnof paleo I quit running! Almost. I did half marathons and such but had a love hate relationship with it. Now I literally do 2-3 tabatta sprints a week. I did 5 k on aug 12 24:10. Hoping to shavena few off in one month to prove distance stinks and sprints rock! Though I may do a half trail for fun with only tabatta training. The anaerobic benefits transfer aerobically. Paleo changed the way I workout too!

I try and keep my intense exercise no longer than 45 minutes. This is about how long it takes for cortisol to rear its mitigating head.

For some info on cortisol, MDA has a decent article: http://www.marksdailyapple.com/cortisol/

Cortisol serves many important functions, including the rapid release of glycogen stores for immediate energy. But persistent cortisol release requires that other vital mechanisms effectively shut down – immunity, digestion, healthy endocrine function, and so on. Among other stress-health associations, the link between elevated cortisol and weight gain has already been established.

And Robb Wolf: http://robbwolf.com/2009/10/12/my-training-and-adrenal-fatigue/

Cortisol competes with testosterone for the substrate pregnenalone. High cortisol means insulin resistance, low strength and slow recovery. Excessive metabolic conditiong makes things worse. What’s excessive? You never make progress, you feel like absolute death doing anything over a few minutes duration. I’ve also had 4-5 sinus infections in the past about 8 months. No bueno.

I'm a recovering distance runner. Now I run 3 5ks and the New Bedford Half Marathon with no training runs longer than 800 meters. For training I do self-styled crossfit and movnat, lots of squats, deadlifts, box jumps, sprints and burpees. Last March I ran the half marathon 8 minutes slower than my PR which isn't bad considering I ran >1 mile per week.

My personal limit is probably long runs of up to about 15 miles, and I routinely run about 35 miles/week right now (no comments about how I'm going to die from too much cardio, please.) I've run the half-marathon distance but I don't think I would ever consider a marathon. I run distance a lot but the longest I've raced is 10K. These days I'm getting more into trail running anyway.

However I rarely do any racing anymore because for me it kind of sucks all the fun out of running. I will quickly become very compulsive and anxious if I start following regimented training plans. Not that I don't enjoy pushing myself, otherwise I'd never get faster, but it's awesome to be able to jut go out and do whatever kind of run I want without worrying that I have a race in three weeks and I really need to focus on that. It's a lot more enjoyable, kind of like how running is when we're little kids, you know?

I ride my bicycle long distances and want to be a long distance runner also. I'm doing a half marathon in October and an ultra in December. I feel great. Also, working a high stress job as a nurse, it cuts down on my cortisol levels big time.

I'd rather stay in my cave and eat some good steak. Running is not for me.

I've done 34 marathons, and lots of utra marathons includeing Badwater (135 miles through Death Valley in July). Of those marathons, so far one has been done since I changed to Paleo (Kona Marathon on 6/26/11). I'm currently training for another marathon and I do about 3 per year. Since going Paleo, I've noticed no changes. I no longer drink Gatorade while running, just water with a B12 vitamin packet and some amino acids (like glutamine). I also do Crossfit about 3 times per week. Any person can run any distance they want, even with minimal training. Our bodies are designed to run quite efficiently (although some people don't necessarily move as efficiently as they could). And remember, it is about 90% mental. If you want to badly enough, you will accomplish anything.

I admit, I don't necessarily like running.

I don't mind 5k's. But I always pick a warrior dash or something that has obstacles or something else to do. So it sort of becomes like interval training instead :)

My max? 10-12 miles. Only because that's how long the tough mudder is, and I'm going to do it this next year. After that, I think I'm going to call it quits on the longer stuff and go back to an occasional 5k with the only training being done is sprints.

Oops. I accidentally logged into the wrong account.