In 2020 I created a simple page with downloadable metronome MP3 files, ranging from 170 to 200 BPM. Each MP3 is an hour long and can be looped in your favourite player. This was sufficient for my purposes, but I (somewhat) recently received some emails from people who used the page not (only) for running, but also for swimming and other activities. They asked for longer MP3's and lower BPM's. I've updated the tool accordingly, and BPM can now be set as low as 60 BPM and maximum duration is increased from 60 to 600 minutes (yes, that is 10 hours).
(Updated) tool for generating metronome MP3's
If you're interested, everything is free and can be found on https://www.reuneker.nl/files/metronome.