Need help with nutrition

Hello,

I recently started lifting, and I don't really know how much to eat. I run around ~35 miles a week, and I lift 2 days on, 1 day off (pull, push, rest, repeat. I don't do legs because I cba and I am already super satisfied with them, please don't try to convince me otherwise). My usual run is ~8 miles long, and my long run is 16+ miles.

My goal is to either eat at maintenance or be in a slight caloric surplus (closer to maintenance, though). The problem is that I don't know if I should eat the same amount of calories every day, or eat more on running days. Also, I don't know how to track my weight, because I technically should be gaining weight from an increased muscle mass, but I don't know how much of that is fat. I also don't do legs, so the increase in muscle mass will be just from my upper body, and that makes it harder to estimate how much muscle mass I gain.

Help would be appreciated. Thanks.