Here's my take...From my understanding, it is living out ones' life in accordance with what Christ commanded and for some of us, that means in accordance with the sermon on the mount.
When I finally figured this out is when the lights turned on. Most of my life I believed I was in a battle between what I thought was my old man fighting against what I thought was my new man. Now I'm not fighting. I moved right in the darn thing. It seems to me one cannot walk in or by the spirit if our old nature (which is dead) thinks it's fighting against our new nature.
I wrote years ago...
Last night is when I took the red pill that released me from the Matrix. I was taught being in Christ meant I was in fellowship with God because I was walking upright in my flesh. I now understand being in Christ is being in the spirit and neither of them (in Christ or in the spirit) has anything to do with the darn flesh. It now seems perfectly clear to walk in or by the spirit is the same as putting on the Lord Jesus Christ. It's the same as having fellowship with Jesus Christ, which is also part of having the mind of Christ. Living in Christ is the same as being strong in the Lord, and in the power of his might. There's a verse where Jesus says without me you can do nothing. Well, I'm never without him.