What kind of death?

[FFC]

The NASV say:

Rom 8:12 So then, brethren, we are under obligation, not to the flesh, to live according to the flesh--
Rom 8:13 for if you are living according to the flesh, you must die; but if by the Spirit you are putting to death the deeds of the body, you will live.

I was reading in chapter 8 of Romans and these verses gave me pause. I see in verse 12 that Paul is talking to christians, so when he says in verse 13 "for if you are living according to the flesh, you must die" what kind of death is he talking about. To make matters worse I was reading from the New Century Version which called this death "spiritual death". Can anyone help me shed some light on the meaning of these verses?

Thanks

[Turgonian]

Notes accompanying the Dutch 17th-century Bible (the 'State Translation'): 'Namely eternal death, as is clear from the life promised here. And this the apostle does not say to cause the believers to doubt their faith, for the contrary he will prove powerfully from Rom. 8:14 to the end of Rom. 8; but he says this to arm them all the more against the flesh and to distinguish the true believers from those, who profess themselves to be believers yet are not such indeed, as they prove with their life that they have not been regenerated by God's spirit, whom he, by this heavy threat, wants to bring to thought and conversion.'

[godslanguage]

I think that verse is talking about death not only in the physical sense, but in the spiritual sense as well. Living fleshly will bring will inevitbly bring about physical death and spirtual as well. Living the way God wants you too will save your spiritual part eternally, you physical body will always die.