With regard to weight training; I would like to clarify what percentage can I use? and second are there any exercises to avoid? For example, some YouTube videos says that leg extensions are bad for runners. What is your opinion?
Avoid nothing. just make sure that whatever you are doing promotes symmetry. Also remember, that you should cycle the weight-training and don’t hit the running hard on the days you weight-train. Weight-training will compromise your muscles on the day you do it, and to then go and run hard will cause injury. I would stick to 7-10 reps and start very light and increase the amount of weight you use each week. You can actually start with resistance training and do no weights at all. All of this will be trial and error. What works for the guy that made the YouTube video may not work for you. What works for another body type, or another experience level, may not work for you. No matter what you do, be calculated and write everything down. You manage what you measure.