Running Macro to fill in new cells without affecting cells already filled

[Salts]

I have a macro that will be run several times throughout a worksheet.
First time I run macro it will fill column AF down to a certain # of rows based information in column A. Lets say in this case row 20.
Then if we need to add more rows below this, I want to re-run the macro so it will autofill down my drop down menu starting at the last filled row and not from the very beginning of the column.

Is there a way to do this? I know my code below works the first time I run the macro, but the second time it overwrites everything I already input



'FILLS DOWN MAILER COMMENTS DROP DOWN MENU
Range("AF2").AutoFill Destination:=Range("AF2:AF" & lastRow)

'FILLS DOWN PRODUCT IMAGE DROP DOWN MENU
Range("AG2").AutoFill Destination:=Range("AG2:AG" & lastRow)

 

[footoo]

What is the value of lastRow?
Is it the actual row number or the number of rows to be filled?