If he's a beginner like I assume from the OP then he'll have a hard time doing HIIT. As a side note you need to not over do it with HIIT, in doing it it's comparable to going to failure on your sets, your body needs time to recover.
This is something you need to assess for yourself OP. Try doing a 3k run and see from there if it's something you want to increase or decrease. You won't want to do the same length run each day if you're doing a big one (do a big one every 2nd day?), think of it like hitting the same body part twice in a row. Alternatively you could just do a small one each day. When you feel like you're nailing the distance, up it. Make sure to warm up properly with dynamic stretches (I also start my run off by walking for a bit) and cool down properly (I spend close to 5 minutes stretching because if I don't I'll be paying for it the next day) and I also walk for a bit at the end of the run. If you do want to train to get used to the idea of HIIT one tip is to use the road lights: when you reach one sprint until you reach the next one, jog til you reach the next one, then walk til you reach the next one. Rinse & repeat.
In reference to the walking vs running, there's a million different opinions out there and for some walking is better, others running is better. Running seems to be the consensus though for the simple reason as you're doing more, burning more calories per hr.
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