Strength and endurance are rivals. “Strength loves rest,” as the saying goes, and it hates endurance. Strength and endurance compete against each other for your training time and recuperative powers. Yes, you can do both (and you should, at least a little). But unless you are a pure strength athlete or pure endurance athlete (e.g. a powerlifter or an ultra runner), you must strike some kind of compromise between the ancient enemies.
Which should you favor? Usually there’s a clear answer that’s dictated by (1) the rules of your sport and maybe (2) your individual game plan.
(1) Gaming the Rules: Most “mixed” sports—not purely strength or endurance—clearly favor one or the other. Middle-distance runners and soccer players are basically endurance athletes who need just some strength training, and vice versa for football linemen and sprinters. That’s determined by the rules of their respective games. Now, if soccer players were allowed to grapple each other, they would have to get stronger. If football had continuous play like soccer, the players would need more endurance. Similarly, if wrestling matches lasted two hours, wrestlers would need even bigger gas tanks. But if matches lasted only a few seconds, or if they had no weight classes, you would get sumo wrestlers.
Horses for courses: different 
rule sets call for different athletes.
In those mixed sports, athletes face a point of diminishing returns for certain kinds of endurance or strength. For example, a boxer needs to bench more than his own bodyweight, but he doesn’t need to bench twice his bodyweight, and if he invests the training time to do so, he’ll neglect his running, to say nothing of actually practicing boxing. Boxers win by boxing. They don’t win extra points for the biggest bench. And no one cares if a shotputter is great at jogging.
Speaking of diminishing returns, in a study of SEAL trainees, sailors who scored the highest in pushups and pullups and certain other measures fared worse overall! The Navy concluded that, in each of their many physical tests—running, swimming, sprinting, pull-ups, deadlifts—they could clearly identify a point of diminishing returns. A max of 70 pushups is not enough, for instance, but 100 is plenty. They tell the hopefuls, “don’t spend valuable time and energy trying to do more. Make your push-up training economical, so you leave time to train the many other qualities important for success in [the first phase of SEAL training].”
(2) Your Personal Game Plan: In the mixed strength-and-endurance sports, the right balance might also depend specifically on your game plan. If you plan to box like Floyd Mayweather, you’d better have a bottomless gas tank, so put endurance first. But if you’re going to be Mike Tyson and assassinate guys in the first round, then for you power comes first.
So what about ruck marching? Should you favor endurance or strength? It’s complicated because so many variables are left up in the air—light or heavy loads? short distances or long? what’s the terrain?—but according to the data, the short answer is …
Neither strength nor endurance! Instead, invest in muscle mass.
Jack Rabbits in Flak Jackets
When Army physiologists studied ruck marching in the early 80s, the height of America’s obsession with aerobics, they concluded that, sure enough, soldiers marched fastest who had the best aerobic capacity. Sure, strength was important too, but the guys who could ruck the fastest were the skinny “jack rabbits” who excelled at running.
Or so the physiologists said. Old soldiers told a different story, though. They claimed that in the field, the jack rabbits lagged behind bigger guys when they were loaded down with a rucksack, a flak jacket, and other gear. Whom to believe?
It seemed significant that the early Army studies tested people carrying light packs for short distances. Under those conditions, it was little surprise that your best runners would shine. But what if you saddled soldiers with 100 lbs. (45kg) (a real-world infantry load) and made them schlep it 12 miles (20km)? That’s what Joseph Knapick and his team did in 1990. And at the moment, those are the conditions that interest me because they come closest to the rules of the game that I’m training for right now, the GORUCK Heavy Challenge.
In that “heavy and long” event, the soldiers who rucked fastest were the most muscular. Not the strongest, heaviest, tallest, leanest, or the fastest runners, or the most aerobically capable, but specifically the ones with the most lean body mass.
Let me repeat that: not muscle strength but muscle mass.
With just one exception, even “skinny strength” did not help. That is, guys who were wiry and strong out of proportion to their bodyweight. The winners were the Dwayne Johnsons of the world, not the Brad Pitts.
Apparently this is the guy to be if you’re carrying a rucksack. 
But I guess I’d still be willing to be Brad Pitt too, if you twist my arm.
I am surprised at this, and dismayed. It’s not the answer I wanted. Regular readers know that I enjoy “skinny strength” training and push it as an alternative to bodybuilding. Normally I only spend a few weeks a year bodybuilding and then go back to my kettlebells and low reps gladly. So this is not the conclusion I wanted, but apparently “facts don’t care about my feelings.”
The researchers measured many kinds of strength (as distinct from muscle mass), and only one made a significant difference: the abs. They tested strength in the soldiers’ grip, low back, quads, hamstrings, and calves, and none of the others was significant. [But there’s a caveat to this in our next post. -Dog in Chief] Only ab strength made a difference.
Strange as that seems superficially, it isn’t surprising. As Pavel Tsatsouline says, strong abs + strong hands = strong person. More and more, kinesiologists are pointing out the importance of what is sometimes called “core strength,” our ability to make our torsos rigid at pivotal moments and resist bending or twisting.
In the case of walking under load, your abs pull your hips under you and keep them there. That way you stay upright, and your legs step quickly and freely. Without that ab strength, you angle forward at the waist and walk bent over. You walk slower that way, and over time you pound the hip joints. So if you can stay more upright, you go faster and stay healthier. That’s what you need abs for.