Running...ever heard of it causing weight gain?

At first, it can cause water weight gain. That usually falls off within 2-4 weeks (so you're still within that range).

Has your food increased? I know when I train for longer distances, I get hungry. I tend to overeat because "hey, I just ran X amount of miles I totally deserve this." Yes, I can eat a little bit more, but I personally tend to overdue it and completely wipe out any deficit I may have created.

Kind of the opposite approach to your question, but I'm going to offer it anyway.

When I started running, I expected to lose weight. But I didn't. My weight stayed exactly the same. Why? Because I wasn't tracking my calories and was eating more to compensate the running. I didn't realize that I was eating more, but I was.

When I started tracking, I started losing. But I had already been running for a while by that point and wouldn't expect to the the newbie "gain". I do see the initial gains/stalls now when I change up my strength training.

It makes sense that you MIGHT gain a little bit of water weight when you start. If your muscles aren't used to an exercise, they will retain some water afterwards to help them repair the little micro-tears that they get from exercising. This is a good thing, so don't worry about it. If you're feeling sore, you will probably see some water weight gain. It'll go away, I promise.

Of course you gain weight when you start running. Expect 2 to 4 pounds of water retention. It really doesn't go away if you're really working to improve your time, but that doesn't matter. After a few weeks, you'll be losing more rapidly than you were before you started running, and this is really the only way I know of to maintain a healthy weight without being hungry all the time and feeling miserable. (Honestly.)

(How do I know it doesn't really go away? Stop running for a week after you've been running and trying hard to improve your time for several months. Cut your caloric intake back by the # of calories you're not burning. . .you will lose 2lbs that week. Neat parlor trick. I've done it on occasion!)

This is the best explanation I've read about the whys of water retention and why you won't stop retaining water.

Add me to the rungry people.

Inexplicably, a 10 mile run does not burn off a whole pizza.

It's a crying shame.

Well, you said when you first started, you did walk/jog intervals and now you're doing run/sprint (I assume) intervals. So it's not like you've been doing the same thing since you initially started, which means you may be retaining water. Anytime you increase the intensity of your exercise, it will put a strain of your muscles which means a retention of water and glucose to help with healing. If you keep kicking it up a notch, you will keep retaining water.

Edited...I checked out your diary and see you don't seem to be eating too much so I don't think that's the problem. Are you logging every little thing? Are you weighing foods?

By the way, congrats on your progress so far! You've done an amazing job! This could just be a little bump in the road. I know it's tough but sometimes this process takes real patience. Good luck!