While I don't use them, this is the sections from the vivoactiveHR manual that talks about how to setup run/walk intervals. http://www8.garmin.com/manuals/webh...UID-228F168D-D601-40E7-9497-AFDB55A5A47C.html
Is it possible you are thinking about the VivoSmart HR and not the VivoActive HR?
Very interesting... no, I was using the Garmin comparison feature on their website. You will see in the image below that the "Interval training (set up exercise and rest intervals)" is not checked for the Vivoactive HR. Clearly, as your link proves, the manual states otherwise. Oh well.
I think that interval training actually refers to something else. When you complete a run with the 235, based on your heart rate it gives you a suggested rest interval to use based on your heart rate. (they are usually like 37 hours and I ignore them, because I run every day except Wednesday, only philistines run on Wednesday )