Is there an easy way to make n guide lines such that each one is x pixels from the previous one, other than manually entering a newly calculated x or y value for each line?
You can also use
Extensions > Render > Grid Creator - this will divide your document into n parts using guides (maximum 10 unfortunately).
You don't actually specify the distance between guides (like what you asked), and it depends on the document width, so Wray's answer is probably better in your case. But it's another way of generating equidistant guides ;-)