Honest question: should you even do cardio if you already lift weights and work a physically active job?

Do whatever you feel is best for yourself.  

I doubt you need additional cardio, given the amount of walking you're doing.  You've probably got that part of your fitness covered pretty well.

HIIT isn't cardio.  If you're huffing and puffing during an activity, the intensity is too high to consider it cardio.  If you're not huffing and puffing, it isn't intense enough to consider it HIIT.

You can add that 1 day a week cardio if you want. No reason not to, but if you feel you don't need it, then don't do it. Even though you walk a lot for your job, that's not the same (to your body) as an intense cardio workout that really gets your heart rate up and lungs working. 

.

Per the CDC some time spent doing aerobic exercise each week is recommended, but the more intensity the less time you need to spend doing it. Examples of how it may be distributed are below:

.

1 - 150 mins/wk (e.g., 30 mins x 5 days) moderate intensity aerobic activity, such as brisk walking + 2 or more days/wk of strength training that works all the major muscle groups

2 -  75 mins/wk of vigorous intensity aerobic activity (e.g., jogging or running) such as 2 sessions of 38 mins + 2 or more days/wk of strength training that works all the major muscle groups

3 -  An equivalent mix of moderate- and vigorous-intensity aerobic activity

on 2 or more days a week + 2 or more days/wk of strength training that works all the major muscle groups

Nope.   30min of physical activity per day is all that is required to keep your heart in shape. 

Exercise has almost nothing to do with significant fat loss or gain,  so extra exercise is redundant,  wastes time and adds to your food bill.