What's a quick way to extend these numbers to a longer sequence, for instance 1 through 20?

elect both cells, and then drag the fill handle Fill handle over the range you want, for instance 18 more rows

Select the range you want, including both cells, point to Fill on the Edit menu, and then click Down.

Copy the second cell, click in the cell below it, on the Standard toolbar click the down arrow on the Paste Button image button, and then click Paste Special.

Noneofthese