okay so somedays i eat a lot, like 3500 calories. i'm trying to lose, so i know that that's ridiculous. if i do enough cardio (say 4.5 hours) to make the caloric deficit, shouldn't i still lose? with that amount of cardio, fitday says i burn 4500 total (bmr included.) mathematically, it should work, right?

Well I don't know your stats exactly, but those bmr things arent exactly the most accurate things in the world. I really see no reason to be doing 4.5 hours of cardio per day unless you really like it. start with a 2500 calorie diet for a couple weeks and if you dont lose a pound or two per week, then lower it like 500 and go with 2000. and remember to do some resistance training as well. Push yourself and youll get a satisfaction of being sore the next day. I would do some research on HIIT to see if you would be interested in that. its good cardio system

i agree....4.5 hrs is too long...and HIIT is awesome....
i was frustrated a lil while ago cuz i didn't see results...but honestly...i started drinking more water and watching the fat and carb content in what i ate and only tried eating a portion no bigger than my fist...then after 2 weeks and a bit...i find that i'm 5lbs lighter....so i reward myself with one cheat day....Hooter's Buffalo shrimp...mmmmmm that was a great night....and now i'm staying focussed again...i heard that shocking ur system like that works to get ur metabolism work more efficiently....i dunno...seems to be working....i think if u eat little meals like that frequently and watch the fat and carbs in it...then u'll see the calories fall and u won't have to do 4.5 hrs of cardio.

Doing 4.5 hours of cardio a day to be able to eat more is actually very unhealthy and gets thrown into the category with anorexia and Bolemia. If you do more than 20 to 25 mins of cardio a day you are burning muscle tissue. This is something that you never want to do. As someone said - a great way to start cutting the calories (I am doing this as we speak) is to keep your portion size the same throughout the day with 5 to 6 small meals. All of my meals now are about the size of my fist and a half. It is working well.