You need to expand in stages, ideally using march moves before the enemy gets too close.
For example, you could do:
ABCDEFGH
to
BD
AF
CEGH
Then in the second move expand column CEGH to
BD
AF
CE
GH
Note that if you initially expand to
AD
AFGH
CE
you cannot then expand GH to the other side of CE because CE is not part of the expanding column (AFGH).
Hence if you have a really long column, it is often only practical to expand on one side.
It is more efficient to use the "turn 90 degrees from a straight column into line" option as that gets everyone deployed in line in one move and you don't need to measure move distances. NB it has to be a straight column, and of course you end up facing 90 degrees away from the original front.
Expanding from a column can be done even if the column has a bend in it, which can sometimes work to your advantage, but sometimes limits your expansion if the bend is the wrong way as it alters the distances elements have to move compared to a straight column.