I’m running more than I am biking at the moment, probably getting about 100 to 120km a month in. I avoid road running like the plague though, its tedious and it bores me to tears, I just stick to trail and fell running, helps I live in the Peak District though as I probably wouldn’t run at all if i lived in the city.
I find it an excellent way of building base fitness and endurance, and the return you get on an hour or two run is the same as going out for twice as long on a bike. There is definitely a technical aspect of running on difficult trails at speed too, not dissimilar to picking a good line on a bike, run with someone who is really good at it and they just seem to move at a really nice cadence over the rocks instead of stuttering through them. I’m good, but I’ve seen lots of others at races who are a whole lot better.