This is the most common usage scenario for devices with wifi-only capabilities: You are at home, at the wireless network. You need to go somewhere so you generate a navigation path there. The only thing you know is where do you go, nothing else. You don't have GPS signal, as you are indoors.
The problem is that the navigation now has some bug and it doesn't take wireless information into account, only GPS. Even though it asks for "confirm using wireless position" or something similar. But after it, it doesn't use it. It starts looking for GPS. The only way now is to actually hack it with "map center" or with creating a temporary point where you are. None of them really easy.
I'd recommend two things:
1. Fix this bug and use wireless just like GPS and how it should work. Rename the GPS setting to "My Location". It's even better as Google Maps uses now "My Location" in the direction planning, making it easier for users to find it.
2. Make the default setting in the From part to be "My Location". Obviously you're most likely to plan from My Location, if not, you are using some advanced track planning way (i mean new track, + point, + point, +, +, navigate).