Hi Mark - you would probably normally just use the original profile that you created the tube from and extrude that profile curve to make a new longer section.

Do you not have the original generator curve saved anywhere?

If not then you can recover it by doing an "untrim" on those surfaces - see here for some more information on untrim:
http://moi3d.com/forum/index.php?webtag=MOI&msg=444.4

Basically if you select those 2 tube surfaces and turn on their surface control points with Edit > Show pts, you'll see that their underlying surfaces are already actually extended how you want - you can recover those by doing an untrim.

When you do a boolean or trim on surfaces, there are new "trim curves" that are created on each surface which mark areas of the surface as being holes or cut away regions, but the original surface is still there underneath all those trim curves and when you do an untrim as shown in that link above you can recover that full surface.

I've attached a version of your file here with those tube surfaces untrimmed and with the original full extended tube recovered. I created this version by selecting one edge of each of those surfaces, and then doing a Ctrl+A so that all the edges on each one were selected, and then pushing Delete to remove those trimming boundaries.

Hope this helps!

- Michael