I am currently in Goldenrod City, near the Radio Tower. I do not think Darkness could fly beyond it and return in a single day, but I haven't tested his flight speed.

[ He vaguely recognises Envy, but still has no idea who Henry's friends are or what they look like, aside from Heather (the Heretic). Darkness knows Envy lives at Shark House but doesn't have a way to communicate that to Walter, and wouldn't do so unless specifically asked. So instead the Murkrow preens himself a little more; if Envy's on the screen, there's a chance Lyra might appear! He has to look as good for her. ]