THIS!!!
I’d say I jog more than run but did an adventure race a few years ago and the trail run was up and down with and overall drop of several hundred meters over 14km.
The specialist fell/trail runners look more like they are in a controlled fall downhill but I found running on a narrow footpath, so having to get my feet in closer to the centre of the path than was comfortable and the overall downhill killed me, I could barely walk the next few days, I looked like a drunk trying to get down stairs for about a week.
Whilst running doesn’t benefit road cycling it is a benefit to mountain biking, and its weight bearing so good for bone density.