I did the same thing before and I'm teetering on that line again. It really sucks and its counter intuitive. I tend to discipline myself right into shutting down my metabolism (aka starvation mode). The truth is that if you are doing that much cardio, you need to eat at least 1/2 of your calories burned or your body will hang onto everything you eat and convert it to fat stores. Also, over time your body will start to burn muscle instead of fat to fuel your exercise and that makes everything worse. One good tip to keep in mind is to make sure you are replenishing your nutrients and proteins vs just getting calories. Good carbs like beans and oatmeal, and fruits are good before a high intensity workout, but protein is where it is at for recovery and long term success. Proteins and Omega 3 fats are your friends, love them and use them to get a few of those extra calories you need and repair your body from the work you put it through. It's not all about calories out calories in.

Congrats on the awesome exercise though. That is an inspiration. I hit the gym 5-6 times a week, but I'm only burning between 300-600 cals with cardio. I add it to MFP and try to not eat LESS than 300 calories under my goal for the day. So far, it has been successfull, but this week I had 2 days that I was 1000+ shy and it bit me in the butt on the scales. Think "healthy lifestyle" and don't beat yourself up. You can do this and you'll be that much stronger for it.