Being a bit of a fitness enthusiast (just a bit) I can tell you that getting your heart rate up for twenty minutes a day is good for cardiovascular health ( not as good as 45 min) but it will not get you in "shape". I am really sick of so called fitness experts (sales people) spreading these bogus stories about getting in shape in 20 min a day. HIIT is good for building speed and burning calories but if you want endurance-ability to tour comfortably, you will have to put on some miles. Now remember a said enthusiast, not doctor, but i think it gos like this. When you work your muscles they build lactic acid so when you HIIT train you build it up than flush it (low intensity) than build it up again and so on. When you just go, there is no flushing time so you need to mork your muscles for long periods of time to develop endurance. or something like that. The old saying, no pain no gain is fact if it doesn't burn you aren't getting stronger. No matter what "they" want to tell you (so you will give them your money)