How can I race against a GPX track? I'm using the Strava app to record segments I ride.  It would be very helpful in trying to beat existing KOMs to be able to upload GPX file(s) to my phone and have audible feedback during each segment to tell me in real time how many seconds ahead or behind I am so I know when to put extra effort in or when I can take a breather.  The audible part is especially important because my phone is stowed in a pouch on my bike and the only interaction I have is via a bluetooth headset. It is also important that the feedback be continuous as some segments are relatively short (under two minutes) and so if I only got a notification every mile or minute or something that would be pretty worthless to me.

I have heard that such a thing exists on certain dedicated GPS devices (e.g. Garmin), but I'm trying to find an existing app for Android that does this.  So far I have tried Record Beater and Ghost Runner, both of which claim to have a feature like this, but have not had any luck.  (One of these programs refuses to import tracks that it didn't create, and the other I could not find any way to import a track at all!)
I used an old app called cardiotrianer for Android. It's old, but should still work for this purpose. It tells you how far in front or behind you are. I don't think you can just read a gpx into it though. You'd need to record the run/ride first. So it would give you the same issue as the other apps you tried.

I would suggest submitting a feature requests for RunnerUp. It' an open source app and the developers are quite active.

https://github.com/jonasoreland/runnerup/issues

I'm sure they would be interesting in adding such a feature.