I, too, _came from_ the dark side (running). Been doing it pretty consistently for 25yrs. Used to MTB aggressively about 10yrs ago, but put that one in the closet for a while. Just joined the ranks of the road biker about 18 months ago.
Observations?
1) In my case, the legs had much better definition when running was the main gig. Road biking has added some girth to the ol' wheels (up a pants size!), but just not as cut. No matter....
2) Originally, moved to cycling because work was just absorbing all of my former running time. Figured cycle commuting would kill two birds;
3) Though I've run for a quarter of a century without a running injury, I _was_ starting to feel my knees while running. Cycling has not only proved to be a good rest for the joints, it seems that--when I do go for a relatively long run now--my knees have been strengthened by the riding;
4) Also pulled the MTB out of dead storage a few months ago, threw some new cables on it, cleaned it and tuned it, and started hooking up with a few dirt friends. I miss it! Definitely does work different muscles, force concentration on different aspects of technique, and get you pretty good at getting quickly out of your pedals 😉
5) Always been partial to running on the trails. Aside from the scenery (deer, bunnies, occasional coyote), it's infinitely easier on the joints. Also seems to do a better job of 'finely calibrating' muscles as you run on uneven surfaces.
I sat down with the running shoes, the trail running shoes, and the SIDI's a few months ago, letting each know that they were loved equally, and hoping to minimize the in-fighting. They seem to have made peace with it 😄