Running

one of three options i can think of really:
a) you are putting on muscle, gaining muscle quickly would be difficult..
b) its water weight. does it fluctuate at all, or hold pretty steady? do you drink enough water, hows your sodium intake?
c) too many calories. I actually have a friend that ran his first full marathon, and GAINED weight in the process. hes always big a big guy, but he gained fat pounds for sure. I see his social media posts, and in his case its all diet. he runs all the time but his diet is total junk.. burgers, pizza, beer, etc.

I try to not focus on the number on the scale alone. as another poster wrote, go more by how your clothes fit, and how you feel. I take several measurements and track them in a spreadsheet. Including several measurements with tape measure, scale weight, and I get a DEXA scan to test my body fat every 6 months..

Were you tracking calories accurately at the time you were running and still gaining weight. For how long did this gaining last?

My body retains water when I run. I honestly don't rehydrate enough. My suggestion would be to go ahead and run, but make sure you're getting LOTS of fluids and keeping sodium fairly low. Also, if running's making you hungry, watch what and how much you're eating... A couple of sweaty miles don't give you a free pass on your diet!

My weight always fluctuates while training for races and my longer distance runs. A lot of it has to do with lactic acid and water retention, but a lot also has to do with the caloric demands of my body after long runs. The biggest thing for me is to make sure that I'm eating good amounts all day long (not storing calories for the end of the day and then binging), being aware and realistic about what I'm taking in and how active I am when I'm not running - sure I may run a half marathon in the morning, but if I sit on the couch for the rest of the day I'm not really burning as many calories as I think!- and adjusting my calorie consumption daily based on my exercise levels.

Mostly I just don't worry about the scale and bring out the measuring tape. After all how I look is more important than the number.

I agree, running isn't a magic bullet. It is part of a healthy lifestyle. If you want to lose fat, you have to deficit calories. Add up your caloric intake and subtract your calories burned for the day (MFP does this for you) and make sure your net calories are at some sort of deficit.

As you exercise the conditions may change (depending on your goals). When I ran 10 miles 2 months ago, I weighed ~200lbs and burned 2200 calories. Yesterday, i ran the same 10 miles and only burned 1537 calories, my weight was ~185.

The only difference is that I weighed less, as a result, my heart (and other muscles) didn't have to work as hard to go the same distance.

So, if i assume that when i run 10 miles that i burn 2200 calories, I would be over-eating and would gain weight.

[EDIT]: I am still learning this myself, but your heart rate during exercise also determines where your body gets it's energy. It sounds like you are running fast and likely operating at a higher heart rate. During that sort of exercise your body is not able to burn as much stored fat for fuel. The slower speeds and lower heart rate running\walking will burn more stored fat for fuel during the exercise.

However, If you deficit calories less than what your body needs, your body will still burn stored fat for fuel when you are not at a high heart rate.

Yep, too much food. If you were running 2 miles at a time, that's going to burn 200-300 calories at the most. Running is a great way to burn calories, but until you get up to longer distances, it's not going to burn as much as you think it is.

Have you taken measurements and body fat or relying on scale and how you feel? Also, since your food diary isn't open, how many calories are you eating? And are you using a HRM to determine calorie burn or another way?