Am I eating enough?

hi everyone-

I am very active, and I either do bikram yoga or run 6 days a week.  I will be starting training for a half marathon in the next few weeks, and I am having trouble maintaining my weight.  I currently weigh 120-121, and I am just shy of 5'6".  I am not looking to lose any more weight, and I am concerned that I am not eating enough in order to train successfully and be so active.  I started tracking my food, and subtracting my calories burned from exercise, I found that I am eating very few net calories (between 500-800).  A typical menu for me:

B: 1/2 cup oatmeal with small banana and 1/2 cup Fiber One (300 calories)

S: 1/2 lite English muffin (50 cal)

L: Salad with 1/2 cup black beans, vegan burger patty, and 1/4-1/8 avocado (300)

S: Apple; sometimes ff yogurt (100-200)

D: Usually stirfry veggies with brown rice, or quinoa, maybe with some tofu (300-400)

Where can I add things in without gaining weight (I just want to maintain)?  How do I know if i'm eating enough to train successfully and stay active?  Thanks in advance!

I don't see how you could possibly be eating enough.  Just roughly plugging in the info you provided, I come up with that you need around 2200 to 2400 calories just to maintain.  I also come up with that  you are consuming at most based on the food you listed 1250.   That seems to me that you are on a definite weight loss program.  Since you are so active AND DON'T want to lose weight, you definitely, in my opinion, need to add things to your diet such as some lean meat such as white poulty meat and 95% lean beef, fish like salmon and tuna, nuts like walnuts and amonds, small red beans, red potatoes, whole wheat bread and pasta, skim milk, egg whites, peanut butter, rasp/black/blue/straw berries, and vegetables like cabbage, carrots, onions, mushrooms, bell peppers, spinach, etc. etc. 

If you stay on your current eating plan and exercise plan, I would be surprised if you didn't lose way too much weight. 

not to be too rude but im pretty sure u posted this already and got a ton of answers saying youre NOT eating enough

honestly i think you will need more as you start 8+ mile runs if you want to perform well at all!!

also if you dont want to drop more weight with the added mileage.

good luck.

In light of your training, I would add a good quality protein shake/supplement to your diet...otherwise, your performance will be poor and your body will burn muscle... I would also increase my carb intake...some shakes have both carbs and protein in them...good luck!