The problem most people have with a workout regime is the consistency. You have one good week of going to the gym or being active almost daily, and then life gets in the way and you spend the next couple of weeks trying to cram in a few workouts in between all the other obligations you have.
Concentrated Effort versus Duration
You may be used to a workout schedule of 1 hour or more when you go to the gym. But the time doesn’t matter – it’s the effort you put forth. You shouldn’t put the bare minimal in your workout just to fill up time, you should feel like you’ve pushed yourself to the limit. This can happen in 15 minutes! All you need to do is modify your workout.