This summer I started cycling, mainly because the public transport in my region was (and still is) very unstable.
To analyze my rides, I'm currently using Runtastic Road Bike, but I consider moving to Strava. Their support for fitness wearables is just so much better.
Now the question came up, how to transfer the data of my already tracked activities. Runtastic can export .tcx files, which can then be imported to Strava.
Drawback: You have to to this separately for every single activity... Too time consuming.
But, lo and behold, you can also export your whole Runtastic profile data in one go (only every seventh day, but that's fine for me).
(alternative location https://git.thesecretgamer.de/faldon/runtastic-converter)
So, if you need to convert a bunch of Runtastic activities to .tcx files, give it a try. It's a very rough solution, with little error handling, but feel free to leave a message or post an issue at one of the repositories.
You will need the script and the template file from the repo. Should work on any of the three major OS, though Python on Windows always feels a little unfitting for me.
And make sure to use Python 3.