The first 10% of effort produces 90% of your result. It takes the other 90% of effort to get that last 10% of result.
This is not a human formula. It is a formula of nature, or reality. Flowing water knows this. burning logs know this. It just is.
When the aliens invade, you do not have to survive for ever. At some point the alien leadership will declare victory and move on. The idea is to out survive the budget constraints of the invading force.
Whether aliens or zombies, the rule applies.
Aliens can get anything from our planet elsewhere. Unless they are just looking for a party, they can get it elsewhere, easier. Invading costs money and resources. I don't care where you come from, it takes effort to take a planet. It takes infinitely more effort to keep it. Zap everyone from orbit or fly around and pick up magic beans round the corner where they are just lying around.
Zombies have to eat. If they don't eat, they simply become very still and really don't qualify as zombies any more. All that shuffling around and moaning takes energy. The laws of physics say that energy has to come from somewhere. Brains have only so many calories.
Be the guy who knows how to filter water, skin a buck, run a trout line. Have a place that no one cares about in mind. Have a backup. Be willing to let others die for you to live. They are part of the 90% I mentioned above. You be a 10% kind of guy. Not the top 10%, the meanest 10%.