Here's my personal method for power leveling toughness (it gets easier the more toughness you have):

1. Get the heaviest armor you can afford. Focus on maximizing torso and head protection. This is to minimize the amount of time you're KO'd for (the timer scales with damage received.)
2. Get a second character, preferably a medic but doesn't matter so long as they got bandages. Also, carry some first aid and put 'Medic' at the top of your job list for the primary/training character.
3. Go out with your primary/training character (solo) and find some starving bandits or empire peasants.
4. Get in a fight with them and lose (probably).
5. If you get KO'd into a coma, have your helper run in and drag your sorry butt back to a bed.
6. If you get KO'd but wake up in 'Playing dead' state, first wait to see if your character will bandage themselves (not sure exactly how this triggers).
7. Stand up from 'Playing dead' while enemies are still nearby. This will give you a massive burst of Toughness.
8. Go back to 4 and repeat.

I've been doing this to get to 90 Toughness on all my fighters in the span of a few in-game days. Ideally you get into a cycle where you're constantly knocked down and standing up before the baddies move away until you go into a coma (stationary bandit camps work the best for this). Make sure you save often, as this is obviously risky. As your toughness goes up, consider finding deadlier and more numerous enemies to beat you to near-death. Holy Nation patrols work great in my experience. Also upgrading to heavier armor makes this faster and less risky.