Hey there Retro,
In my humble opinion you're putting the cart before the horse here when you try to go from 4 to 13. The reason I think so is that, in the process of overcoming hindrances in the first four steps you will learn a great deal about your mind, and about inconsistency. You need not run off prematurely to another exercise in order to observe this, as anicca is evident from the earliest stages to the last.
If you focus on those first four, looking for ways to overcome this hindrance or that, then you will eventually overcome them. When you do, you allow yourself a mental 'pat on the back', and feel good about your accomplishment. This can lead nicely into joy which, when attended to along with steady focus on the breath, becomes stronger. The whole experience becomes very pleasant, and you can relax into it.
Now, you say that you don't get much time to meditate so you try to get the important work done. That is a trouble I can appreciate. But I don't think that focusing on samatha is a waste, nor do I think you would be commended by the teacher for trying to jump over the intermediate steps. If you practice all the steps in order each time you meditate, then you will become more adept at them. So what might initially take 40 minutes will eventually take 20, or even 10-15. Then you will have a pleasant practice, the mere thought of which makes you want to go and sit.
You will also be in a much better place to observe anicca, but on a more subtle level than just that you get while contending with the arising and passing away of the hindrances. And no, I seldom sit more than 40 to 50 minutes.
So seriously, don't go putting the cart before the horse.