Eating exercise calories too much?

If you're serious about training for a marathon, you probably shouldn't be trying to lose so much weight, they are 2 opposite ends of the spectrum. This is just my opinion of course, and isn't stated in an attempt to shoot down your efforts, it's just a statement of fact.

Training for an extreme even requires focused technique that does not (and should not be) included in a weight loss regimen. Now if you just have a goal of completing a marathon distance (whether it be in an organized race, or just on your own) then that's different, so I think some clarification is in order.

Someone with the physical ability to run 26.2 miles (without stopping, walking, or resting) is probably not (IMHO) in a position to safely lose 2 lbs a week as you have to be in at least moderately good shape to do so, which means your body most likely won't have the energy stores (fat) to be able to lose 2 lbs a week. If you CAN run 26.2 miles and you ARE losing 2 lbs a week, I contend that a portion (and most likely a rather large portion, say at least 25 to 30% of your weight loss) is muscle and other lean tissue. Which is probably not a good thing.

As a personal trainer, and runner who has trained and competed in many medium to long distance races (I'd guess at somewhere around 50 to 75 races in the last 15 years, although most were in my college days when my knees could handle 2 to 3 races a month and training) at a competitive level (my 10K time is decent, or used to be, my best 10K was 38 minutes and 25 seconds, which put me slightly ahead of the curve of male running times).

this isn't to say you can't run a marathon while still trying to lose weight, but in order to train safely and effectively you need to fuel your body, and that means dialing back the weight loss a bit. Multitasking is great, but competitive running is not closely aligned with weight loss. Your body will, of its own accord, seek a level which makes training easier, which will naturally lead to weight loss, but forcing the issue will hamper training efforts and could lead to injuries and/or weakening of your immune system. I can't tell you how many runners I know that came down with the flu right before a race because they were training super hard and not feeding themselves right.

This is all just to emphasize that there's more to weight loss than just a calorie deficit, to make it stick, my belief is that you must take into account every aspect of your life and work your nutrition into that routine. If you work out hard doing HIIT type routines 4 or 5 days a week, you need to make sure your electrolyte levels and fuel (usually in the form of complex carbs) are high enough to sustain it, if you weight train to failure multiple times a week, specific eating patters and nutrition levels (of things such as protein and carbs) should be monitored closely, if you're a straight cardio person, carb and potassium levels need to be watched...etc.

So should you eat your exercise calories when training for a marathon? Probably IMHO, you at least need to come close in order to maintain proper levels of calorie expenditure, it's a bit of a guessing game at the beginning because everyone is a little different, but trying to do too much is almost never a good thing.