if you want to keep up your cardio while gaining weight I would consider interval training a few times a week. I personally think that interval training is more suited for most sport conditioning and certainly fighting anyway...

We use a mixture of three energy systems and running for an hour is good for endurance at the training pace. Fighting is on and off and to goal is to be explosive as well as endurance for the fighting time etc..

I would do interval training, say you were training for a 3 min round and two minute break I would do that in my cardio... the three minutes would be varying intensity (think about how you would like to fight for three minutes and how you are likely too)..

Say a fast run for 30 seconds, run for a minute, fast for 30 seconds, sprint for 30 seconds and then fast run for another 30... (just an example) mix it up how it seems fit to you...

Then take the break time to work on practicing recovery. I think focusing on recovery is a good practice for fighting. Teach the body how to relax and recover as fast as possible...

Then go for the next round...

I am also a believer in keeping intensity high and building endurance as opposed to starting with low intensity and high endurance...
Focusing on intensity is training the right combination of energy systems and better training the muscle type fibers that you want for fighting...

I have training tri-athletes who tire in one round... the energy systems don't really cross over that well and training endurance is training slow twitch muscle fibers which are not explosive...

I would do two minutes with high intensity and only two rounds (then build) than 5 rounds of low intensity...

Training this way also won't be eating your hard gained muscle like running a marathon would...

Bag training along similar lines would be even more accurate... the energy used is related to the muscles used and running is not the same as kicking and punching etc... It can be a nice suppliment but I think very over rated in terms of duration and frequency.

When I started training this way my starting goal was to do two laps in three minutes (with varying intensity) but I think the breaks are great for many reasons... good practice, trains more closely the right energy systems... great time to mentally prepare... I have found it a big help to tell myself I am recovering and that each round will be better than the last and that I LOVE IT and choose to be doing it. I have also found it powerful to have a positive thought... a beautiful or funny thought or memory that always makes me smile...

I think it is important to focus on fighting/training but I also think that we should be able to get to the place we can turn it on and of in a heart beat... Focusing on the fight (stress) during recovery time will just keep us in a more stressed space... calm, relax, recover, calmly consider the next round, take 2 seconds to have a positive/funny thought and truly focus on breathing and building energy back up...

In training I feel it is very powerful to focus on teaching self how to recover.

Why do we have breaks in fighting?? Recovery time.. is it not worth learning to recover just as much as learning to get work? It is in my opinion...

This way we burn less hard earned muscle and more closely train the energy system for a real fight.

IMO&E

In terms of choosing interval times an expert in physiology and sports conditioning may have better numbers but really you want to match it as closely to fighting as you can. How do YOU want to fight? Where are you know? Keep intensity high and work towards that. Also how do most fights seem to go (maybe ajusting a little for the type of fighter you fight and or looking at how you typically fight as well as how you want and just match it to your goals as much as possible..

for pro (Thai style) its 3 on and 2 off... I believe in training that way..

I would shorten breaks but only when that allows intensity to remain where I want it or my heart rate has dropped low enough...

Its not what you can do its what you can recover from. Just cause you can jog for an hour doesn't mean you can fight for 3 minutes... Train as close to how you fight as possible is how I see it...and save that hard earned muscle and keep the explosiveness... IMO