Cycling along GPX track
There are lots of sources in the Internet, where cyclists can download GPX files with cycling tracks.
Navigating along the track requires primary an audio guidance, which informs about turns and warns when cyclist leaving the track.
Leaving the track ist mostly a wrong turn of the cyclist, so he wants to return to the track immediately.
A warning could be spoken after 30 to 50 meters away from the track.
Optionally, an alternative route can be calculated. This should be decision of the cyclist.
Perhaps he wants to reach an intermediate target which makes it necessary to change the route a little bit.
But this is secondary, the main purpose of the app should be to help cyclist stay on track and do this with audio guidance, so (s)he does not need to look at the screen often.