We walk as those who have been raised from death into life. I believe the passage in 1Thessalonians 4 is talking about physical death and resurrection. The stunt, of course, is to determine in all cases when Scripture is talking about physical life and death and when it is talking about spiritual death and life. It is difficult, or almost impossible, for us to understand what it meant by eternity, since we live in a time and space continuum. Really, eternity is a description of Who God is.
Eternity transcends time.
We catch glimpses of the meaning of eternity as we walk closely with Jesus and begin to know the Father through Him. When we receive Jesus, we break out of time and into eternity. God gave us eternal life and this life is in His Son. Eternal life is not dependent on our physical life, it is not dependent on time, it is not dependent on the limitations of our earthly existence. It begins when we accept Jesus, and it continues beyond time and space limitations.