I do this. I train strength twice a week and run once a week for cardio.
My main goal is to get stronger but I also want to maintain and improve my conditioning.
I use a pulse watch that estimates my Cooper test distance. It has been slowly increasing. I am now at 2650 m on the Cooper test, at age 45 which according to the web is "very good".
What I do is natural intervals. That is I run a ca. 1 km circuit in a nearby park.
The circuit has a flat, a uphill and a downhill part. I use the flat and uphill part as my interval. That is I run fairly fast on the flat and then try to maintain this speed until I barely reach the top of the hill.Then I catch my breath jogging downhill.
I usually run for 30-45 minutes.
By conditioning most people mean Vo2 max. Say you have to run for the bus. Do your loose your breath and how fast does it take for you to recover? Interval training is the most efficient way to increase VO2 max. Natural intervals are more fun than using a stopwatch.
Remember to warm up by running in moderate speed for 5-10 minutes first. Also if you have not been running for a long time; you better start out running in moderate speed the few first weeks.