I don't have any knowledge of the MapMyRide app I'm afraid. You can download routes from plotaroute.com in various formats, including TCX, FIT, GPX and KML, so if the app has a facility to import files in either of this formats, it could possibly provide what you need.
There are a number of apps that other members of our site have used for navigation. OSMAnd seems popular and also Track Navigator.
Before I ask any question, please know the following:
I really like the idea of creating a route on plotaroute.com and uploading it to MapMyRide so I can follow it.
Thanks in advance for your help.