My shoes look like they have under 100 miles on them, but they are over 300, there is def ware on the bottem, should I let them go to 400 and over? Is there actually risk of injury if you wait too long to replace shoes? Most of my miles are on trail, but I do a fair amount on pavement (25 percent i guess)
Definitely replace them when the soles have holes in them and you cannot glue the soles back to the shoes anymore. So, If you go over 3,000 miles to that point, that is adequate mileage.
You can wear them until they are uncomfortable because they are so compressed on the soles or the uppers rip. I've put over 1000 miles on a pair of shoes before.