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I don't do the miles all at once, MWB.
I guess I do about 20/30 miles a week. I replace my shoes every 6 months or so ... which is about 600 miles of use.
After that, they still feel okay, but they're not doing their job properly ... and that's when you can damage yourself.
I don't have too much fat, but I'm "athletic" rather than slim. Also, my bum's a bit ... ("muscular"?) ... from running on the beach. My friends joke that it looks like an outboard motor, cos it's a bit prominent. It looks good in jeans though, LOL.