I used to buy running shoes the same way I bought regular shoes: find my size, try them on for a minute, and call it a day.
The problem is that running isn't walking.
As I started increasing my mileage, I noticed my feet were constantly sore. My toes felt cramped. My arches felt tight. Some mornings, my heel would hurt so much that the first few steps out of bed felt awful. I blamed my training, recovery, and even my running form. The last thing I suspected was my shoe size.
What nobody tells new runners is that your feet swell during runs. A shoe that feels perfect in a store can feel completely different after an hour of pounding the pavement. Your toes need room to spread. Your feet need room to expand. When they don't get that space, little issues start piling up.
When I finally sized up, the difference was immediate. No more cramped toes. Less foot fatigue. Less discomfort after longer runs. The funny thing is that the larger size initially felt too big. I had spent years thinking that a snug fit was the right fit.
Now whenever someone asks me about running shoes, I tell them not to obsess over the latest foam or carbon plate. Start with fit. Your feet are doing all the work. Give them the room they deserve.