Tired shoes are tired for a reason and that is because they are old
People should be comfortable about the idea of letting these shoes go and buying new ones. Old shoes can actually harm your body and this can cost more in medical bills than actually buying a new pair of shoes.
Walking and running shoes normally last for about five hundred miles
The mileage may vary somewhat depending on the types of terrain that you walk on while wearing the shoes. Footwear can last longer when used on gravel, a trail, and a track than when they are used on a pavement. Polyurethane soles are much more durable than an EVA midsole and these factors influence the longevity of footwear.
You can check to see if your shoes are still fine to wear by performing the press test
Push the outsole with your thumb upwards towards the midsole. New shoes will show that the midsole compresses into wrinkles and lines. In time the midsoles will compress less even though the same amount of pressure is applied. When there are heavy compression lines on the shoes after the press test has been applied, then you know there is not too much cushioning left.
With regards to the general wear and tear of shoes, the dirt is not really an issue as you can always clean the shoes. Broken heels and holes in the soles are better indicators of how long the shoes are still good for. Some shoes will mold to the shape of your foot and this indicates that they have been over used.
If the Shoes Don't Fit
Any aches and pains in the feet are also indicators that there is very little cushioning left in the shoes. Pain may not be limited to the feet as you may experience pain in the back, hips, legs and knees when wearing old footwear. The formation of blisters indicates that the footwear has stretched and your feet are moving about too much in the footwear.
Many runners prolong the lifespan of their shoes by buying two pairs at the same time and then alternating them on different days. This is a very clever way to extend the longevity of the shoes and will actually save money. You can apply the same rationale to casual or work shoes.