No offense taken, don't worry. But it has nothing to do with how good a runner you are. It has to do with how much (or really, how little) cardio training is needed to produce fat loss.
Think of it this way: If someone has never lifted weights before, but they wanted to gain muscle, would they need to jump right into a 6 day a week training plan than involved high volume, complicated exercises, and super-high-intensity techniques? No. They'd need to start with something basic. The same thinking should apply to your cardio training.
HIIT should pretty much never be done post-workout. If you want to do High Intensity Interval Training, I'd start by doing it on two of your off days. 15-20 minutes, working up to a 3:1 work/rest ratio depending on weekly progress.
I wasn't sure of the equipment you had available, so that's fine.
I'm not following here. Is everyday either 50g carbs or zero carbs? That's crazy-strict, and if it's the case, then I definitely suggest two or three higher carb days.
Actually, I'm really, really hesitant to give you more info to read because I think you're close to overthinking things already, but give this article a read and see how much sense it makes:
That would definitely be a solid plan for what you're looking to do.
Your calculations are off. First you say you have 50 grams of carbs every other day, then you're saying that a 400-500 calorie difference happens when you "just remove carbs", which would mean you're dropping 100-125 grams of carbs on your off days. Either something's getting lost in translation or one of us just isn't getting their point across clearly.
Here's my favorite question: Thinking back... what, exactly, did you eat on your last training day? And what, exactly, did you eat on your last off day?
I prefer introducing cardio with easy-ish to moderate intensity work right after weights (as I said earlier). But... if you want to do HIIT, I'd start with it on two of your off days. Right now, there's no need to do HIIT and post-workout cardio.