There are sites that tell you the calories you consume based on weight, activity and distance/time. Google of calories burned running will bring them up.
These are only ever a rough guide but helped me when I decided to throw in additional running etc. There are so many things that cannot be fully accounted for in a simple calculation.
You can gain weight as a runner, I did it, but you will have to accept a likely drop in pace over distance. You will also need to increase food intake as you increase weight.
Finding training slots can be interesting too as you need to try avoiding over-training if you want growth and that is all but impossible.
When initially increasing intake you will either be force feeding, stealth eating or both as I did. Stealth eating is things like adding oil to your food, or one I used was using a very wet version of oats and milk instead of water, literally to force my body to adapt to more food. Now I eat enough for a small family and it's no effort. System I use is eat as long as possible while awake, and advise on eating while asleep is welcome.