I'm surprised nobody has posted a thread about this.
I have a Samsung Gear S3 Frontier which has been borderline transformative for me. When I go on runs or go to the gym it's my phone, music, and wallet (all of which get left at home). I'd imagine that Apple will polish that experience significantly. Assuming that Apple progresses the fitness side (or a 3rd party like Wahoo does it for them), I could see this being a viable substitute for your Garmin.
A lot of people will probably poo poo the idea of running with anything but some cash in your pocket and a key under the doormat but I always have to have a phone on me for work. Also (though I haven't tried it yet) I could see it being handy at the end of races when you're trying to find your loved ones (I can't imagine the "no cell phone" rule will be enforced here).
jens: This is aero-shaming. And it's not OK.
Murphy'sLaw: We shame because we care.