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These are my Mizuno Enerzy. They are only a few years old, but they survived my 110kg to 75kg, 5km in 50 mins to 10km in 55 mins fitness journey. Its getting a bit embarrasing at the gym when people see the duct tape. Ive already stitched the holes up three times.
How long are running shoes supposed to last you? They are expensive! Are they expected to do this, or should they last much longer?
You've gotten a lot of great responses already. Basically, the weak point in a shoe is the foam, and you start to feel the foam go before you see it. That's one reason it's nice to have a few pairs of shoes to rotate through. You can run in one pair, and the next time you do a similar length/time in the other pair, and you'll be able to start to feel when your ankles/knees/back or whatever get sore for no reason.
In terms of the distance you get, it really depends on speed, your weight, your technique (heavy heel strikers probably wear through shoes quicker), and the surface you run on. Possibly the biggest factor is the amount of midsole foam, though. The foam stilts that are popular these days may only last you 400 km, while a more typical shoe gets 800 km.
Like another commenter said, "barefoot" shoes can last a very long time. You probably don't want to go all-in on those unless you want to do some more homework on them, but it's something to consider if you are budget constrained.
What I do now is just stick to a brand that I know works for me. Shoes are built on "lasts" which are basically a foot-like form. Traditionally, they'd be a wooden thing that a cobbler would wrap leather, etc, around. Now, it's basically just the 3D foot model. If you get 2 shoes built on the same last, they are going to fit your foot similarly, so it's not so bad to buy online. They make new model numbers every year or so, and when a new model is in, the old model goes on sale. When they got 50% off, I'll buy a couple pairs. Obviously that only works if you have the money/space to do it, but it is helpful.