The Donner party had to literally chop their way through woods and up mountains. Often making just a couple of miles a day. And then to have the snow come the day they were climbing the final summit and have to turn back. Having lost most of their oxen and few supplies left it had to be a bone crushing disappointment.

IMHO that is one of the lessons. That your ability to hang on long term and endure in the face of repeated disappointment and setbacks can be as vital as all your material preparations. The ones that made it had a lot of grit.

That and when taking a long trip in a group it's always a good idea to set aside supply of "fava beans and a nice chianti". No reason not to be civilized about it.

Two lines that caught my attention were:
Don't take any cutoffs and hurry up every chance you get.
If I don't see you again; do the best you can.