A lot of it will depend on where you need to go and what you need to bring for any given emergency. If you're going to bug in, then not much is needed. If you're bugging to a place that's five miles away, or plan on traveling five miles a day, and you have everything you need in that pack, you're probably ok. Your daily caloric intake will affect your abilities as well. I guess what I'm getting at is that there are so many variables, there's really no way to say what the optimum level of fitness is. Try to be as healthy as you can.
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It's not that life is so short, it's that you're dead for so long.