I walk 5 to 8 miles per day at my job. understanding that my hr is not in the cardio zone by mere walking, is there any cardio benefits?

Yes, but...: It is definitely more beneficial for your to have a job that is active rather than sedentary; however, you wouldn't want to count this as your exercise routine. Fit some truly aerobic (elevated HR/break a sweat) and strength training into you life for significant health benefits.

In short, yes. : Even though you do not reach your cardio zone HR when you walk there is still significant benefit from being active. Walking as little as 30 minutes per day has been shown to improve overall health and decrease the risk of many conditions, including heart disease. You are not increasing your overall cardiac endurance, but there is still cardio benefits.