Is there an easy way to add ____ until the end of the line?
With tab stops.
Select your text.
On the ruler at the top of the document, click the spot where you want the underlines to end, to add a tab stop over there. It will look like a funny shaped "L".
Now double click on the tab stop that was added, and under Leader, choose option 4 which is an underline. Click Set and then click Okay.
Now on the end of each line of text, press the TAB key to add a "tab" which will fill up the space until the tab stop, and it'll be underlined since you chose that option.