F___ around and find out. Literally.

Do that tempo run the day before your long run.  Maybe do that 20 mile long run the week before your mountaineering objective.  Maybe don't worry about bringing enough food on that 20 mile run.  Seriously though, how can you REALLY learn without making some REAL mistakes.  With the ultimate goal of adaptation to an objective in mind, how can you possibly know how you'll feel when the big day arrives?

Hope for the best, plan for the worst and do everything you can to be ready.  But along the way, don't be afraid to F___ around.  You will find out.  Probably disregard that if you're going for an FKT or world record or something.  For myself and having no natural ability and having to train really hard just to have a chance, experimentation has been key in finding out what works and what does not.

Forgetting gear, getting off route, not sleeping enough, bonking... None of these sound like fun.  I'm not saying to set yourself up for failure, but when shit happens it forces you to adapt.  Adaptation to the objective you're planning is the purpose of training.  All training is, is an experiment.  Have you ever stayed up too late the night before a long run?  Lession learned, right?  Okay, in my case maybe not.

I've pretty much made every mistake in the book.  I have an excellent track record of learning things THE HARD WAY.  When shit happens, you will adapt and you will build character.  Take some risks in training and get out of your comfort zone well before the big day.  I can personally varify that the more you F___ around, the more you will in fact find out.