Learn English – “Hence the need to make X” vs. “hence the need for X”

The differences between mix and mixture are subtle.

A mix of something is often descriptive of the make-up or components of a mixture:

The castle was built from a mix of sand and water.

A mixture, on the other hand, is the thing that is mixed:

The cake mixture is often a mix of eggs, flour, and oil.

There is a lot of wiggle room in usage and the distinction of when to use one or another is fuzzy. A general rule of thumb is to use mix when naming the individual parts, and mixture to describe the whole.

In the examples you gave, I think both would be acceptable, because you are either talking about a mix of hiking and camping that make up the trip, or the trip's "mixture" of hiking and camping.

It appears that this construct used to be a lot more popular in the 1800s and continues to be used today. Going by Ngrams:

Excerpts: Barbara Heyman, 2012:

I looked at it later in 1922 with the idea of arranging it for violin, cello, and piano. Hence why we still call it the “Trio”.

Thomas A. Blackson, 2011:

The details of his pursuit were so undefined it was difficult to understand how the pursuit was supposed to work and hence why anyone should value it so highly.

Marcel Fafchamps, 2004

This may explain why the threat of stigmatization is largely ineffective against so-called informal sector firms which, as a rule, are not registered — and hence why transactions among informal sector firms remain quite unsophisticated.

In other words, this usage appears to be quite valid.