The other day I was trying to download a route from MapMyRide and I was following the instructions but it just wasn't working. I tried again and again and nothing. I googled the issue and I got the solution.
The problem is that MapMayRide uses an incompatible GPX file extension, so the trick is to convert it to the version supported by Garmin. Here is the full link with the answer
But the simplified version is this:
I hope this helps