"So it is written: 'The first man Adam became a living being', the last Adam, a life-giving spirit. The spiritual did not come first, but the natural, and after that the spiritual." NIV translation
Paul reminds the Corinthian believers that the physical body came first through Adam as he quotes from Genesis 2. All people on the earth began through God's creation of Adam and Eve but this was only physical. Eternal life came through the last Adam (Jesus Christ) who gave us the Spirit.
"The first man was of the dust of the earth, the second man is of heaven." NIV translation
We are reminded that Adam was created from the dust of the earth while Jesus Christ was sent down from heaven by His Father. It seems like we spend a lot of time and efforts on preserving this earthly body but the simple fact is that it was created out of dust and will return to dust. We are to keep our minds focused on the things that will last such as our spiritual health.
"As was the earthly man, so are those who are of the earth; and as is the heavenly man, so shall we bear the image of the heavenly man." NIV translation
Although there are variations in the size, features, etc of people, we all have many characteristics that are common. These characteristics have been passed down to us from their beginning in Adam and so we resemble Adam while we are in this earthly body. It does not matter what we do to maintain this body, it will wear out and and return to the dust. When it does, we will get our new body that will resemble that of Jesus Christ.
"I declare to you, brothers and sisters, that flesh and blood cannot inherit the kingdom of God, nor does the perishable inherit the imperishable." NIV translation
Even the most fit and beautiful earthly body is not good enough to get into heaven. Why? The simple answer is that these earthly bodies still have the sin nature in them due to the fact that they descend from the earthly Adam.
There are those who use this verse to say that we are not going to have a body in heaven but that is not what Paul is saying. He is just saying that it will be a new and different kind of body without the sin nature and its effects.