If you have an old Android phone laying around that can run at least android 5/6, there's lots of good tracking applications that can run persistently and create a .gpx file written to disk in a location of your choice under /sdcard/. May not even be necessary to purchase a smart watch if you don't want heart rate.
One of the things I particularly like about that app is that it's fully configurable for how often you want it to get a GPS fix (has a direct impact on battery life), what filename prefix to create and where to create it, whether to create a new file every day or every session, and many other toggles and knobs in the options.
GPSLogger (the one walrus01 linked to) is a gem. I've been using it continuously for 7 years. Not all location trackers are the same.
It lets you log with cell tower but fall back to GPS if the accuracy is poor. Like walrus01 says, it's very configurable in how accurate you want it to be versus battery usage, or if you want to optimize it some way (like not record if it hasn't been moving).
While there were monthly crashes in the first two years I used it, the past 5 years it's been rock solid (running 24/7 with maybe one or two crashes for basically 5 years). You don't want to discover that one day the app hasn't been logging data for the past couple days. In comparison, the Android OS probably crashes about once a month for me.
It can automatically upload to various cloud storage places (Dropbox, Google Drive, but even FTP) so you can generate charts on a server with a script.
Truly open source, low battery usage, saves in multiple file formats, and the developer is active on Github with issues. Making a background mobile app is not easy because Android is constantly trying to reduce background battery drain or background process spyware.
https://www.gpxsee.org/
If you have an old Android phone laying around that can run at least android 5/6, there's lots of good tracking applications that can run persistently and create a .gpx file written to disk in a location of your choice under /sdcard/. May not even be necessary to purchase a smart watch if you don't want heart rate.
I recommend this one: https://play.google.com/store/apps/details?id=com.mendhak.gp...
One of the things I particularly like about that app is that it's fully configurable for how often you want it to get a GPS fix (has a direct impact on battery life), what filename prefix to create and where to create it, whether to create a new file every day or every session, and many other toggles and knobs in the options.