In “You May Be Strong…But Are You Tough?” (10 Jun 2014, Imminent Threat Solutions), Khaled Allen writes:
Men in particular often confuse toughness with strength, thinking that being strong is automatically the same as being tough, when in fact the two are distinct qualities. As Erwan Le Corre, founder of MovNat, says, “Some people with great muscular strength may lack toughness and easily crumble when circumstances become too challenging. On the other hand, some people with no particularly great muscular strength may be very tough, i.e., capable of overcoming stressful, difficult situations or environments.”
Toughness is the ability to perform well regardless of circumstances. That might mean performing well when you are sick or injured, but it also might mean performing well when your workout gear includes trees and rocks instead of pullup bars and barbells. “Toughness…is the strength, or ability, to withstand adverse conditions,” according to Le Corre.
Being able to do that requires both mental and physical toughness. No amount of mental toughness alone will keep you from freezing in cold temperatures, but if you’ve combined mental training with cold tolerance conditioning, for example, then you’ll fare much better.
So, train in the gym if you need it or love it, but also get outdoors. Run, lift, carry, throw, clamber, walk, hike, climb, crawl, move on varied, complex surfaces in varied, complex terrains full of obstacles and in a range of temperatures and climactic conditions. Be fit for reality, not merely for a gym.