Sitting does two things: it causes your hip flexors to do work all day, and it prevents your gluteus maximus from doing anything at all.

Doing this for hours on end trains the body tries to use its hip flexors for everything. Standing, lifting things, walking, even slow running. The hip flexors aren't meant to do this, and they are connected of course to your back muscles.

But the back muscles activated in concert with the hip flexors are not the "right" ones. They contort your back and cause imbalances and eventually back pain.

Running CAN help undo this.

But at certain paces of running (very slow ones that aren't much more than walking), the butt still is not used well, and you're not working to undo the 8+ hours of training you've given your hip flexors.

The more vigorous the running, the more your "prime path" evolutionary pathways of hamstrings, gluteus maximi, and lower back will be activated and trained.

Of course the older you are, the harder it is to do these paces, either sustained or speedwork.

Straight leg deadlifts can also help, but still isn't as good training for undoing the sitting position.

I'm not a doctor, but running can definitely work out your back. Long trail runs with lots of elevation gain leave me with a sore lower back, mostly because "leaning into" the hill keeps my lower back muscles working to maintain posture. I've only experienced this when pushing lots of mileage on big hills, though differences in posture may result in different sorts of runs triggering this. My running buddy has complained about this on much shorter runs.

It's entirely possible this is useless for preventing back pain, or may even be bad for my back, but I wanted to share.

There might well be a benefit in the lower back, but the scapula and trapezoids are particularly sensitive to desk work , and running is not going to help them.

Can you share your "illustrated guide to back exercises, aka core strength exercises" ?

No, it's long gone :-) But you can find a lot on youtube. Many of them are in the P-90X "Ab Ripper X" video. It's mainly lying prone on the floor and lifting both ends of your body 25 times, then leaving the ends on the floor and lifting the center. Repeat on left side, right side, top side, bottom side.

The hardest one for me is lying face down, arms extended forward. Lift your chest and legs off the ground. Oww! These are called "Supermans" because you look like Superman flying through the air.

It'll give you a vast new appreciation for when a dancer lifts his partner overhead by the waist and she gracefully lies prone only supported by his hands on her waist.

I double-dare you to even get your hands an inch off the ground!