cardio measurement

What is a good way to measure cardio improvement? I'm not looking to measure it in an absolute manner, such as to compare it against someone else's, but I want to have some kind of metric to determine if I'm improving over time. I'm not sure what a good metric for this would be.

Do a one mile time trial once a month or so and keep track of
your times. If you don't have access to a track, find a stretch
of road that is about one mile long and just run that exact
same route.