I would take walk breaks as needed, but make sure you're taking scheduled walk breaks. So if you decide you're going to walk ever 5 minutes and then take a 1 minute walk break stick to that exactly. No more, no less. If 5 minutes ends up being too easy of a workout then you can increase it but you won't really know until you've done all the intervals. I would do at least 5 intervals.
And every week I would add one run where you increase your time. So for example if you do 3 runs one week with 5 to 1 run:walk intervals then the next week do 2 runs at 5:1 intervals and one run at 6:1 intervals. The next week do all your runs at 6:1 intervals. Then go to 7:1. Does that make sense? You could also try looking at the Couch 2 5k program and start at whatever week you feel matches your fitness levels.
If you're getting stitches then yes, you are breathing wrong. Stitches happen when you aren't getting enough oxygen. Make sure you keep your shoulders relaxed, your abs tight and take nice, even breaths.