Mortal bodies are not the sin nature, AKA the flesh, that fights against the Spirit. The Spirit of God is not physical and the fight is not like an arm wrestling match with the body. Paul was not physically crucified when he said "I am crucified with Christ". The reason why uncrucified flesh sins is because the sin nature is embedded in the carnal husk over the soul...it is invisible but ACTS on the body. Paul said, in Romans 7 BEFORE being crucified with Christ...it is no longer I but sin IN me. He was not talking about his body, but the sin nature that covers the soul...called the power of the flesh.Our flesh is our physical body. It will be resurrected when the Lord returns for us.
Rom. 8:10,11 (W.E.B.)
10 If Christ is in you, the body is dead because of sin, but the spirit is alive because of righteousness.
11 But if the Spirit of him who raised up Jesus from the dead dwells in you, he who raised up Christ Jesus from the dead will also give life to your mortal bodies through his Spirit who dwells in you.
Those who are Christ's have had their flesh crucified (not physically) in order to take on a new covering...that of Christ.