It may be the longer distances you are running. Your muscles are holding / retaining the water for repair.
From what I understand, you're running 10+ miles now, so that may be the reason why as I am sure you're eating habits have not changed /increased. I am not sure if it's normal, but I've noticed that since I've upped my milage, the weight is not coming off. I haven't gained yet, but it's at an almost standstill.

I'm only 5'3.5" Amy, so i know what you mean when you say 5lbs is a lot on a short person. I am very close to your weight so I know every pound counts. I still think your body is holding on for repair purposes. We're puting our body's through a lot, so it's just doing its thing so we can do our thing.
I vote you / we forget the number on the scale for now and just concentrate on training. I'm not saying to not weigh /check in, I'm saying not to put so much emphasis on what it says as long as you know you're eating and training right.

im training for half marathon also !

I think water repair is likely one culprit. Also, are you eating enough on long run days? In order to net over my BMR I end up having to eat around 3000 calories on long run days. I will likely put weight loss on hold once I get close to my half (which is September for me) and eat at maintenance so that my body gets plenty of fuel for the half.

I totally just came on this site to find answers to this same problem! I'm in week 5 of my 1/2 training and I've gained 2.8 lbs. I'm about to lose my sh** b/c I was really hoping to drop at least 10 lbs over the course of 12 weeks! I'm only up to 8 miles in my training and I'm really discouraged to the point that I may quit. I'm in my sister in laws wedding in October and I needed to drop weight, not gain it. This is super freaking me out at this point. I'm 5'5" and 162.8 pounds now. So not happy. I have my calories set at 1567 and to be honest, on the weekends I'm not good at tracking so maybe that's my problem? I know about the muscles and the retention with soreness so I expect that and I also expect to drop a pound a few days after but jeez!!! Why run and burn 2500 calories a week (or whatever it is) and gain!!!

Because it builds cardiovascular fitness, gives you nice calf muscles, and it's fun!

but jeez!!! Why run and burn 2500 calories a week (or whatever it is) and gain!!!

Oh boy, I wish running was fun!! Could care less about calf muscles!! LOL! I will give you the cardiovascular fitness though

yep i gained about 2kg and also seemed to lose some muscle. However that is more because i decreased my strength workouts as my mileage went up.
I don't think i was eating quite enough carbs for the long runs either.

Since then i have learnt that it is so important to fuel your body sufficiently and with the right macros!

So glad to have found this so I can be prepared for a potential gail. I just started my half training this week for a race in January, Yeah it's a long train, but it's my first half! I'm 4 lbs from goal, but have a great trainer that has taught me to not focus so much on the scale and more on my fitness level. I'm going to be watching this closely, and also watching to assure I don't use my training as an excuse to over eat.

I have read that while running long distances or doing constant/consistent run workouts your body sort of goes into a "cruise control" like on a car. Your body is minimizing its fuel use by maintaining the same speed no matter how much longer you run. AKA you aren't burning like you think. The key for maximum fat-burn is to switch up the workout by varying speed and intensity. Try sprints followed by a recovery walk or running on different levels like hills. I add in yoga during the week and weight-lifting (BodyPump) to mix up my workout schedule. If I could run everyday I would but from my research, in order to successfully train, you need days of rest and other types of training... not just running. Hope this helps! -Training for my first half and whole marathon Hope I do it!