Depends on your goals. So you mention marathon training. Endurance cardio.

So, P90X is high intensity cardio with a strong strength focus, except you fail because you are just plain tired of high reps. It'll make you stronger though. It'll train the anaerobic and high end aerobic system. It'll make body respond by storing more glucose. That is good endurance.
But it doesn't train the body to use fat better so that the glucose doesn't have to be as much.

Lifting is overloading the muscles. The response is to make the stronger. More so than P90X would.

Marathon training is endurance aerobic, again training to store more carbs, but also to use fat for longer and better.

The lifting and marathon training would be a hard schedule. You would have 1 day available for a long run (which is actually all you need), but that would wear you out for the next day's lifting, if using the same muscles. So that is ineffective.

A couple of the days after lifting day could be a nice shorter below aerobic HR zone run up to 60 min. Shouldn't harm the repair to lifting done the day before, shouldn't tire you out too bad for the next day lifting.

And the lifting, especially the leg stuff, will help the running, and is like your fast run replacement.

So you could lift 3 times weekly, each time proceeded by running, the first time a long aerobic level run. It will impact that first lifting session, as muscle will already be tired, so just skip the legs for that session.

Now, P90X and training, you could have P90X replacement your short intense runs, plus added benefit of strength and agility training with it.
Any running the day after could probably be up into the Aerobic range, just one run. Other should still be low aerobic for that training. And then long run should have rest day before it and after it.

So 3 x weekly for that too, but couple sessions back to back. followed by low aerobic run, another session, rest day, long run, rest day.

You will get tired and need a break every few weeks, that's a lot. And stick to 15% deficit, as lots of cardio is a risk to losing muscle mass, so don't want to eat to little.
And want to get that TDEE right for the deficit.

In fact, I'd count the long run as 60 min for the TDEE levels, even though you plan on going longer. Eat the extra time and calories back after the run, you'll need them for the next day's workout.

Maybe you can help me Heybales. I have been stuck at this weight for quite sometime now. I haven't lost a pound or an inch and I have been using the p90x workouts. I don't follow it perfectly, but I do most of the strength training workouts and the cardio twice a week= 5 workouts per week. I haven't been doing yoga because...frankly...I hate it. I am already at a healthy weight (5' 9" and I weight around 150-155ish). I looking to drop a pants size and tone up. I am passionate about fitness and I want to reach an optimum and maintainable fitness level. I am a teacher so during the school year I work a lot of hours and take classes as I'm working towards my Master's. What do you think would be best? Should I continue with P90x and just lift heavier (8-10 reps instead of 12-15) or should I give NWOL4W a try? I know NWOL4W takes cardio out for a few weeks but I can't give that up either because I enjoy doing it!

For one lifting session a week, you could skip the lifting for the lower body, and do true HIIT workout first. That'll strengthen the true running muscles.
Have to be outside though, can't do HIIT on a treadmill.
That's for when you start training for marathon in earnest.

For a good month, I'd say do true lfiting 3 x weekly. During school when busy anyway, short intense effort will leave you with more time.
Do gentle cardio on day after. P90X is not gentle cardio. It's also not true lifting even if strength training.
It's very stressful on body, and if too much deficit which is also a stress, may be hard pressed to get any loss.

I am in my off running season right now, I like to run in the fall/winter months so I am doing NROLFW now then in Aug I'll start training for my 1/2s I want to do towards the latter/end part of the year. I did that last year lifting heavy and training for a 1/2 at the same time and it was counterproductive.

I got a smack on the head and was told to focus on something so my goal for this year is to focus. lifting in the first half of theyear and running in the latter half of the year. I am 2 months into NROLFW and after having done P90x I couldnt't stick to it. I like NROLFW becuase it is only 3 days a week which is awesome and I can lift heavy.