i was built pretty much just like you. 5 8 130 when i was your age. I have gained about 25 pounds of muscle since then. But that was 3 years of working out most of the time. No suppliments some whey protien but really just working out and eating right. My biggest issue has always been injuries. As soon as i start feeling strong again i get hurt doing other stuff. I hate doing half workouts cuz im injured.
I would say gaining 20 pounds of muscle that fast without something like roids isent gonna happen. Its not really needed either. In the beggining you will gain muscle fast but everybody levels out and has to get over humps. The most important thing is to get a good plan and stick to it, as well as eating right every day. Also, make sure you switch up your workout every 6 weeks. Your muscles start to get used to exercises and you will stop gaining. As you said streching is def important. Especially if you plan on gaining tons of muclse. Also try dynamic warmups instead of streching, before workouts or skiing. Still strech but only for flexability not injury prevention.
I would say gaining 20 pounds of muscle that fast without something like roids isent gonna happen. Its not really needed either. In the beggining you will gain muscle fast but everybody levels out and has to get over humps. The most important thing is to get a good plan and stick to it, as well as eating right every day. Also, make sure you switch up your workout every 6 weeks. Your muscles start to get used to exercises and you will stop gaining. As you said streching is def important. Especially if you plan on gaining tons of muclse. Also try dynamic warmups instead of streching, before workouts or skiing. Still strech but only for flexability not injury prevention.