I started running fairly casually but eventually started doing marathons. I run about 110 miles a month so I'm switching out shoes every 3 months. My feet suck so I'm also replacing a pair of orthotics every 2 years. I got a full set of summer and winter running gear and enough of each to last me a week. I also have a hydration vest, a GPS watch (not necessary but very useful), a set of Aftershokz bone conductive headphones so that I can listen to music while maintaining situational awareness, a pair of good sunglasses specifically for running (I sweat a lot and would not want to ware the same pair for everyday use), I have a few soft flasks that go in my hydration vest, and I've had to invest in a pretty decent treadmill to run on during the worst parts of winter.
Not everything I've listed is 100% necessary but as with most hobbies it finds ways to become expensive the more dedicated you are to it.