First, note that your solution doesn't really work.  Consider this test file:

    $ cat test1
    Network
    Administrator Network
    Administrator

And then run the command:

    $ sed '
     s/Network Administrator/System User/
     N
     s/Network\nAdministrator/System\nUser/
     s/Network Administrator/System User/
     ' test1
    System
    User Network
    Administrator


This solution does work:

    $ sed ':a; /Network$/{N;ba}; s/Network\nAdministrator/System\nUser/g; s/Network Administrator/System User/g' test1
    System
    User System
    User

We can also apply this to your `guide.txt`:

    $ sed ':a; /Network$/{N;ba}; s/Network\nAdministrator/System\nUser/g; s/Network Administrator/System User/g' guide.txt 
    This guide is meant to walk you through a day as a System
    User. By the end, hopefully you will be better
    equipped to perform your duties as a System User
    and maybe even enjoy being a System User that much more.
    System User
    System User
    I'm a System User


The key is to keep reading in lines until you find one that does __not__ end with `Network`.  When that is accomplished, the substitutions can be done.

### Multiline version

If it helps clarify, here is the same command split over multiple lines:

    $ sed ':a
        /Network$/{
           N
           ba
        }
        s/Network\nAdministrator/System\nUser/g
        s/Network Administrator/System User/g
        ' filename

### How it works

1. `:a`

    This creates a label `a`.

2. `/Network$/{N;ba}`

    If this line ends with `Network`, then read and append the next line (`N`) and branch back to label `a` (`ba`).

3. `s/Network\nAdministrator/System\nUser/g`

    Make the substitution with the intermediate newline.

4. `s/Network Administrator/System User/g`

    Make the substitution with the intermediate blank.