Hello, welcome to the forum.
You will want to use the PRESSPULL command.
After starting the command you will need to click inside one of your profiles to select it, then PULL it in whichever direction you want to lengthen it.
There is a good video on this site in the BASIC 3D TUTORIALS section about the EXTRUDE & PRESSPULL commands.
It only shows entry level uses of the EXTRUDE command, but is a good starting point for BASIC modeling.
PRESSPULL is particularly well suited to easily creating orthogonal holes and voids in 3D solids.
This one video will really be an eye opener for you, and totally appropriate for working with extrusions as you do.
You should also work your way through the rest of the 3D Basic video tutorials on this site, to help you hit the ground running.
As you start to play with 3D, I would suggest that you use an ISOMETRIC perspective, especially when creating and modifying
3D entities, which you can access on the VIEWS toolbar or panel on the ribbon.