Running alone is time to yourself. The headphones, your breathing, a city still asleep or already turned in. You do not want to clutter that with a phone to check or messages to send. And yet dawn and night are exactly the hours when you would like someone to know, just in case.
The trouble with the usual answers is that they ask something of you at the worst possible moment. Mid effort, you are not going to stop and let someone know, and you will not think to write anything if an ankle goes at the far end of an empty park. Safety that depends on your vigilance abandons you exactly when you need it.
Which is why the mechanism that actually works here runs the other way round. Before you leave, you start a solo trip with a time you expect to be back. You put the phone away and you run. If you are back in time, nobody is disturbed and it all quietly disappears. But if that time passes with no sign of life, your contacts are told and your last known location goes with it.
The principle has a rather grim name, the dead man's switch, borrowed from trades where people work alone in dangerous places. The idea is a good one despite the name: your safety does not depend on an action you have to manage to perform, but on an abnormal silence that acts on its own. You have nothing to trigger. It is the absence of news that does the triggering.