How many calories do you eat!?

Hello blog, it's been a busy few weeks!
For those few of you out there reading away--or not, because I haven't posted, I'm back with a quick check-in!

I am back with some questions:
1) How do you know how many calories you should be eating?
2) If you are or in the past were eating too few calories, how did you bump up those calories?

So for example did you...
A) Just start eating larger portions?
B) Eat more snacks?
C) Have more delicious cookies and ice cream?

Did you bump up your calories over a period of time? How long?

OR
Did you just realize you needed to eat more--and so you started having two dinners instead of one?

Here's the thing of it:

Right now I am running 40+ miles a week, on the way to 50. I just realized through a series of events that I am probably eating not enough calories--as in--today I ate 1300 and then ran off 450 of those.

How many calories are appropriate for a woman running this amount? Yes. I realize this makes me sound somewhat dumb, but is it--2500? 3000? 1700? I mean how do I find out the "Optimal amount?"

I'm curious to get some other perspectives on these questions..yes, I'm googling around ;D