In the quest to keep Z engaged in some kind of exercise, even if it isn't mt biking, we tried running this year. Started off on the rail trail, which is OK in that it's flat and the chance of an ankle twist is low, as are the elevations.
It's also boring.
I've sorta been trail running for years, but it's been random bursts while out hiking. So we gave it a try a few times on the trails we normally bike. He wasn't a huge fan. I like it.
I think it's similar to mt biking in that your brain and body can't "phone it in", they have to be in a constant state of awareness and coordination. It's like juggling, the more you do it the more the peripheral system learns to react and adjust for you. I don't have a plan or a training regimen, just get out a couple times a week with a friend or alone and see what I can do. Often the start of the run is horrible, like my body has totally forgotten coordination. At some point a circuit breaker resets and everything is hooked up the way it should. A ways into it and I marvel that I'm not stopping for breaks, even on the climbs, just a steady rhythm of propulsion over ever changing terrain.
It's certainly a different set of muscles and exertion. After most of the runs, which are still usually under four miles, the whole body is feeling it. I've had achilles issues (shoes?) and even after that cleared up the calf muscles are usually the first to put in a one star review.
Not sure what Winter will bring. With fat biking I've developed a set of clothing and gear to follow the temperature below freezing. This might work for running, except that I seem to put off a lot more heat and sweat later in the run, which would indicate that I need to get better at layering. which means some way to carry the extra stuff.