What does in mean to walk by the flesh?
Romans 8 is a great example of the stark contrasts you see in Scripture. This chapter gets very in depth in its contrast of "walking according to the flesh" versus "walking by the Spirit.
When you walk according to the flesh, your mind is set on the flesh. Your mind is set on gratifying the flesh. Your mind is "set on" all things "SELF", you are serving self. This "mind set" is hostile to God, it CANNOT please God.
The reality is, YOU CANNOT do what pleases God when you "walk according to the flesh", it is IMPOSSIBLE - it would be like a 2 year old trying to play chess, they haven't got the mind for it. The difference is, the 2 year old can grow up and learn the game of chess; but the mind that is set on the flesh needs to be changed by God. God must open your eyes to the Truth, He must illuminate your mind with the Gospel, He must give you His Spirit.
When God does this, you have now been given the "ability" to think differently, you can now "set your mind" on the things of the Spirit - you can now walk by the Spirit. It is only when you walk by the Spirit you can have peace with God, you can now do what is pleasing to God.