Following procedure is I think the easy one to do the auto-filling large number of cells, no mouse required :)
step-1: Fill 2 or more cells with the required pattern (maybe with some formula), select and drag
(few empty cells) to check if auto-filling is working as per your requirement.
step-2: Select the first cell of your intended sequence (already filled one).
step-3: Press 'F5' (a 'Go To' window will appear), and type the last cell number till where you need to fill (for
ex; A20000). Now, do NOT press enter alone, but press Shift+Enter to select
all the cells from your first cell.
step-4: Press 'Ctrl+D', all the cells selected will be filled accordingly.
Tip: This procedure can easily be extended to multiple columns; each with different pattern or formula. Select the first filled cell of the leftmost column in step-2, then, select the last intended cell of the rightmost column in step-3. All columns will be auto-filled at once by pressing 'Ctrl+D'. This is a very useful shortcut if you use excel a lot.
Please let me know if there is an issue.