If the old Adam was crucified with Christ, why is he still with us?
Although God reckons the old Adam as crucified with Christ when one believes, that reckoning is not yet full experience. The old sin nature remains present, pursuing and plaguing the believer and warring against the new divine nature until one leaves this mortal body.
Les explains that the believer has two natures: the old nature, that old Adam who constantly pursues us, and the new nature given through the salvation experience and union with Christ. This is Pauline doctrine: the old sin nature is in constant warfare against the new divine nature. Dying Daily means the old Adam must be kept in subjection and put to death. In Romans 6, especially verse 13, Paul deals with reckoning old Adam as our mortal enemy and constantly putting him to death. God, in His omnipotence and sovereignty, reckoned our old Adam as crucified with Christ the moment we believed. He is dead, but in experience he is still with us; he plagues us until we leave this body of mortality.