mission [1]:

    (1) a specific task with which a person or a group is charged
    (2) a definite military, *naval*, or aerospace task

So in answer to your question, yes. If hikers wanted to call their hike a mission it is well within their rights. Merriam-Webster isn’t going to stop them.

You should seriously find better things to do with your life than obtuse rebellion against dictionary definitions. It makes you look like an illiterate nonce.

[1] https://www.merriam-webster.com/dictionary/mission