Honestly, hardtack isn't too bad, as long as you don't look at it like a meal. It's either an ingredient or an activity.

I do Napoleonic era reenactment, and that means I've made an eaten a fair bit of hardtack. Pictured here is the fancier, more modern sort, with holes so you can break it without using a tool. The alternative is just round-ish chunks of it, which make you happy the muskets of the day came with a nice, solid wooden stock.

If you treat hardtack like a meal, it sucks. You can't chew it, so basically you're just swallowing it whole after grinding it up, and that sucks both going in and coming out. If you use it as an ingredient, say for stew or porridge, it works just fine. Not even hardtack can remain solid after boiling it for an hour, and it's an amazing thickener for pretty much everything. If you grind it up enough, you can even view hardtack as a form of solid flour, and bake pancakes with it.

The other method is to treat it like an activity. Eating hardtack is like chewing gum, something you can do while walking for a few hours. Keep a piece in your mouth and sloooowly let it soften, and at the end of the march, you'll have gotten some free calories in.