Or did I just miss that chance?
For those of you who don't know about this Pokewalker Route, you obtain it by using GTS at least once in Heartgold/Sousilver.
No. There is no possible way to unlock that route now, since you can't access the GTS anymore.
You can use that link to not only get event exclusive routes but also access the GTS. It involves changing your console's settings to connect to custom servers (Wiimmfi), which substitute for Nintendo's discontinued services.