Theres lot of ways to operationalize it (and differing opinions on what the 20 should look like) but if you're just trying to improve mile time (while maintaining your lifting schedule) I'd aim for 3x 30 min/week with the last 5-7 minutes going hard so that your heart rate is already elevated

You can do that by feel but especially when just starting out using heart rate zones to guide the effort is v helpful