Most of your runs are going to be guesstimations based on how fast you think you typically run and how long it takes you to cover one of your courses on average. When improvising a course, you’re relying totally on what general pace you think you’re running that day. And that’s fine—it really doesn’t matter if what you say is your 10-mile loop is 9.8 miles long, or if you call a run 5 miles when it’s longer than that.
Online mapping tools can be useful to see how far your courses are. Don’t get too hung up on their degree of accuracy—I’ve seen the same program give a different distance for the same course on consecutive days. They also aren’t going to measure a course exactly as you ran it, and they don’t account for things like hills and wind. So be content with getting a close-enough measurement.
When in doubt, err on the side of underestimating. It’s more common to think we’re running farther and faster than we really are than it is to shortchange ourselves.