I am a skinny cross country runner in high school looking to gain some muscle for next season. I would prefer a workout that I could do at home but I do have a gym membership. I don’t want to gain so much muscle that it will slow me down, but just enough to make sure my muscles don’t tire during a 5k race. Please help!

i switch on and off between nights, everything other night i run on a treadmill and the nights i skip running i workout on a bowflex machine.
i workout not to loose weight, i workout to gain muscle for lacrosse season.
tonight i ran and before i ate a plate of pasta, and had a large glass of protien shake.
after i am thinking about having frozen rasberries

any tips? or any suggestions?

I can’t afford a gym, I also have a treadmill at home, besides the treadmill, will running up and down the stairs from basement to third floor be a good workout, there are about 25 steps.