The Headspace Journey was designed by Andy to be done in sequential order (Take 10, Take 15, Take 20, The Discovery Series and so forth...). The reason for this is to enable you to get a solid and consistent practice on the go.
Much like the syllabus of any course, each programme builds upon the last. By doing the programmes in order you will be able to confidently glide through the Headspace Journey thus feeling the most benefit from the practice.
The good news is that after you've completed a session, you can at any time go back and re-play it. This counts for sessions within the pack you're currently in, or even any previous session in another completed pack.
This will allow you to develop your Headspace practice, or even go back and re-do shorter sessions if you wish.