Thousands Of Cells Added To End After Formula Run

I'm using the record macro function to make a code that is concerned with formatting information generated in excel by another software program.

My original idea was to replace the information in column F with the concatenated info of column F-L, but I couldn't get this to work either. So now I'm looking for a work around for my work around.

I can get the macro to work just fine with "Macro 1" below, but unfortunately when I use the entire column range, it populates thousands and thousands of rows below the (around 50) lines of information I have. However, the populated information in my sheet is completely variable - it could be 1 line or 5000 lines. These will also be printed, so I cannot just cap the range at 5000 because then it would print all of the blanks.

I tried several work-arounds after googling. I tried to leave the blanks and then delete them once they were populated - messy, and it crashed excel. I tried End(xlUp) and I tried using lngLastRow = ActiveSheet.Cells(Rows.Count, "A").End(xlUp).Row

how to only copy the formula in Column M to the last row of information (which would be the row with the last value in Column A).

MACRO 1:
Columns("M:M").Select
Selection.FormulaR1C1 = _
"=CONCATENATE(RC6, "" "", RC7, "" "", RC8, "" "", RC9, "" "", RC10, "" "", RC11, "" "", RC12)"
Range("M:M").End(xlDown).Select
Columns("M:M").Select
Selection.Copy
Columns("F:F").Select

[code].....

MACRO 2 (Only the Changes):

ActiveSheet.Cells(Rows.Count, 1).End(xlUp).Select
Range("M" & lngLastRow).Select
Selection.FormulaR1C1 = _
"=CONCATENATE(RC6, "" "", RC7, "" "", RC8, "" "", RC9, "" "", RC10, "" "", RC11, "" "", RC12)"
Range("M" & lngLastRow).Select
Selection.Copy