I think you will be disappointed by your results with running. In order to understand why, you'll need to know where the weight would need to come from.
- The number one class of people who have problems gaining weight are ectomorphs.
- Ectomorphs burn fat quickly and easily, so the only way to gain weight is to build muscle.
Exercise is a specific thing, what you work on will be what gets stronger. More importantly, whether you specialize in endurance vs. power output also affects how your body adapts. For example, endurance would be running a marathon and power output would be sprinting in the context of your question.
The only way to build muscle is to give it work that is more than it is used to. Your body will respond to prepare itself for that level of work. This is why, especially in the beginning, you will need to keep increasing the amount of work your body needs to do. This is a fundamental principle in strength based exercises, whether you lift weights or perform body weight exercises.
The problem with running is that it is very good at burning fat, and most people who run specialize in endurance. That means you won't be getting significantly stronger, and your upper body won't be getting much work at all. In short, the amount of muscle build will be in your legs and won't amount to the 8kg body weight you want.
Resistance training is where you will get the most bang for your buck so to speak. If you have a weight set at home, then I highly recommend getting the book "Starting Strength" by Mark Rippetoe and Dr. Kilgore. It covers proper technique (no need for a trainer), as well as how the exercises work the muscles in your body. The routine outlined in that book will make you strong, which in turn will help you gain the weight you want.
If you do any type of running (a form of conditioning), then I highly recommend specializing in sprints or interval training as it complements the type of energy use that your body is working for with resistance training.