you have another 3 defenders to cover you plus another one from DM (assuming you play with 4 at the back).if you+your CB partner couldn't intercept the ball,one of the solution is to ask the other CB to at least close the space for the forward to make him hard to score/cross;and you+your CB partner can go to the center and cover that area.

i play as CB though.me and my team always back each other and cover the space when either one of us left our area.

my 2 cents.

it's not always about who outrun who.the main thing is to prevent him from scoring/crossing.try to bring the forward as far as possible from the goalpost.