I've tried many of those exercises and I agree that you can easily do 15 minutes the first time. Consistency is the problem I found. When I started at 30 seconds and built up slowly, I found it much easier to build a habit and do it consistently. Again I use the weightlifting analogy - I can squat close to my body weight, without any warm up, when I'm out of shape. But I can only do it once and I need a week to recover and I'm likely to injure myself. If I start with the empty bar and increase weight gradually for a month, I can work out every other day and hit my target consistently.
