Emily, I have come to the same conclusion as you. I like running, but it's overall so much harder for me. I live where essentially, there is no flat, so it's never easy. I can pretty much handle 3 miles, with hills, but I am not sure if I can or will do more. My legs just feel like cr*p if I go to ride the next day, which happened yesterday.
I am not going to stop running and I do envy those of you who can go out and run 10 miles and then ride 50 the next day. Riding is too much a part of my life, in a way that running isn't that it's always going to come first, even if I am getting older and slower.