I just started doing HIIT last week. I'm 39 yrs.old and I usually put in 40+ miles a week. I've been running for 10 years, not competitively but just to stay in shape. The older I get it seems like I'm putting on weight around the mid-section so I decided to try HIIT. I'm currently doing the 30/30 routine. I warm up with a light jog for 10-12 minutes then I sprint for 30 seconds at 90%+ MHR then light jog for 30 seconds at 50% MHR. (I'm up to 12 intervals for a total of six minutes.) Then I cool down with a 10-12 minute light jog. My question is, I'm doing HIIT 2-3 days a week, on the other days can I do my regular 6+ miles a day or am I over doing it? Will doing my steady state running on top of HIIT defeat the purpose of doing the HITT and cause burnout? I feel like I need to do something else on the days I'm not doing HIIT. Any advice would be appreciated.

just listen to your body. as a seasoned runner, you can handle more than the average joe, so i'm not sure where the burnout might begin. if you start feeling sick or sluggish, then you will have to ton it down.

a great way to increase your fat burning is to hit the gym. muscle burns fat very well ... maybe take a couple days of the week and do some simple weight routines.

You're lucky in that your body has adapted to run 6+ miles a day. You obviously put in a lot of base miles to get there and now your body doesn't have to work real hard to maintain that.

However, you're also unlucky in that your body has adapted to run 6+ miles a day. Because it doesn't have to work so hard and because your metabolism is continually slowing (I should know, I'm 48), the same amount of food eaten is going into excess.

I think the conventional wisdom among coaches is that you need the steady state base miles to get to the level for competitors. If you are not competitive (running, duathlons or triathlons), then you can use your base miles as a springboard to do intervals while running in combination with HIIT.

As for the HIIT, see the stickies on it. I was thinking 30 seconds is a little short duration - the HIIT I do on the spinning bike is generally 45 seconds to 1 minute.

I doubt you will overtrain by adding a long run or 2 as well as doing your HIIT. There is nothing better that the high you get after a long run, why give it up just because you are expanding your training horizons? Listen to your body. I personally think that overtraining has been spouted so much recently that everyone is paranoid of it.

I disagree, for fit people I always hear best results are from short 1:1 intervals usually around 15-20 seconds.

That being said, variety is always key to avoiding plateaus - so mix it up with your intervals.