I'm just going to say that MotionX is the easiest way to do what you want to do. Step by step...
1. Make up your route in Google Maps and copy the link.
2. Go to this website....
Paste your GM link and hit convert, and then you download your GPX file.
3. Just email your GPX file to email@example.com
and it will email you back a file that you open with your iphone and it imports to Motion X.
You are done, go ride!