so basically just judge when appropriate
if your rcb moves in behind your rb, you need to push your lcb closer to where your rcb normally would be, and you need to pull in towards the center of the field. the very few times that i've had to play in the back, we've had tactical "walkthroughs" in practice to address shit like this. i would imagine that you would do something similar
of course, as a defender, you aren't isolated. you should receive help. just communicate and use your discretion
ok when i go for a run should i cut in with the ball or always down the wing?
depends upon your team and what the other team gives you. don't have a one track mind. your coach should give you an idea of what he wants you to do
let's say you're right footed playing on the left and can shoot from distance. if it's there, cut in and take a shot from time to time. say you have a striker who has been destroying the other team in the air. if you can get down the line, look to get there and center the ball in the air.
if it's not there, don't bother to force it.