cat x* > big-tar-file.tar
Assuming you used the default naming scheme of split.
Replace the red with your actual files.
When you plan to restore your files make sure you have enough drive space.
The splitted tar files plus the final full version will require about 40G of drive space. After you've created the full version, you can delete the splitted tar files.
I prefer using rsync when backing up files, and when used with the delete
option, it makes a mirror copy of the source and destination folders.
Be careful though, if you misuse the delete option, it can delete files you didn't want deleted. See the man page for rsync or google it.