Obviously the main goal of Max-OT cardio and HIIT is to maximize fat burning in minimum time.

I'm not sure that is my goal with cardio. I have high blood pressure (usually about 140/80) and my family has a history of it. Heck, my dad has been on blood pressure medication since he was 25 years old.

My goal with cardio is heart health. I want to lower my BP and gain endurance.

Is there a way to structure longer cardio sessions (over 25 minutes) so that they both help my heart and minimize catabolism?

In order to not "burn muscle" but burn some fat during cardio you have to pay attention to 2 things, stay below your Lactate Threshold heart rate and also make sure to not deplete your glycogen stores.

These two things are important because if you go anaerobic (above your lactate threshold) you can ONLY burn glycose. (So no more fat burning.) Your body CAN convert amino acids into burnable sugar so you might lose muscle.

Now if you deplete your glycogen stores most likely you will just bonk. You may start burning up muscle also before you are completely out of sugar.

In theory if you keep your heart rate under the LT and you keep eating quickly digestable carbs during the workout, you should be able to go forever (well, until you get too sleepy/tired/etc).

So if you want to do long sessions of cardio, make sure to eat enough carbs (that will make your glycogen stores) and do not go above your LT for too long.

How do I determine my Lactate Threshold?

There are basically 3 different kinds of methods to measure your LT heart rate.

You can go to a lab and they will measure it for you most accurately. Most of us will never have this done though ;)

You can also do tests at home or in a gym with a helper and get a pretty accurate result.

Check this article out :

http://www.cruciblefitness.com/etips/bike-lthr.htm

If you do not have a power measuring device (like stationary bike or elliptical trainer or something) you can just use the preceived effort, speed and your breathing.

One thing I did for my own test was that instead of using a preceived exertion scale of 6-20 I just did a 1-10. At the end of the test I converted the 1-10 over to the 6-20 scale. The reason for this is that when you are about to roll over and die during the test it will be very hard to think in 6-20. ;)

One more thing that the article does not mention but is helpful is that typically people cannot do more than 5 "iterations" after they reach their LTHR. So if you did a test with increasing power/speed every minute and you lasted say 25 minutes, you have a pretty good chance that you passed your LTHR somewhere between 20-25 mins.