If you haven't lifted weights at all, start and keep up with protein intake etc (shakes if you have trouble eating enough). You should get those sweet sweet beginner gains and put on some easy weight.That's been my go-to since I moved back home after graduation. Basically doing something similar to what I was doing while in XC back in highschool
Ok. Yeah, since I don't really have access to the University gym anymore, that's all I really have access to at the moment. Once I actually start getting money back into my bank account, I'll probably sign up for a gym here in town.
Lifting is great but you can definitely put on weight with the proper diet and calisthenics. Pound protein and clean carbs. 3-4 days a week hit a variation of push ups, sit ups, squats, planks, leg lifts, pull ups, chin ups, etc. If you do these properly and consistently you will put on weight. Just worry about doing as many or more reps than you did the day before. I lost 100 lbs and was really skinny, then I reshaped my diet and kept working out the same way and put on like 15 lbs of muscle. And I don't lift. So just let me know if you got any questions