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This is bad advice. If you want to be mentally checked out during a ruck march, do the following:
Get a bottle of magnesium citrate. Make sure it’s as cold as possible. Chug it down then follow with 24-36 oz of water no later than 30 minutes before your ruck march.
The average person is lacking in magnesium and this will make sure you are not only hydrated but in the best possible frame of mind for your 12 miler.