When you find a shoe that fits well and have run in it comfortably for several weeks, make a note on your calendar to purchase another pair about a month before your marathon. You want to make sure the shoes you run your marathon in are fairly fresh, but not brand new.

Generally, running shoes should last about 300 to 500 miles, or three to four months if you’re running 25 miles a week. It’s good to log your weekly miles and make sure to replace your shoes once they hit this point, even if the uppers and soles look in good shape. The cushioning and stability in shoes breaks down around the 300- or 500-mile mark, but these aren’t visible changes. If you continue to run in shoes that are too old, you may become injured. Heavier people should replace shoes more toward the 300-mile mark, while lighter folks can usually wait until it’s more like 500 miles.