There is no definite answer to this question as it depends on so many factors, however in my experience one thing is for certain that running or starting running in battered, worn out shoes is the leading cause of running injuries and can be as bad for you as running in shoes that just don’t fit you.
Running shoes are a consumable so even though you may have had them for a long time but not run many miles in them, you do need to be mindful of whether they are doing the job of protecting your feet/ankles/knees/hips as with any other consumable they do have a shelf life. However, they are generally more expensive than many other consumables and people do treat them as long term products and tend to hold on to them for a long time.
Trainers are designed to have a shelf life, if you think of the cushioning of the midsole of a trainer as being similar to bubble wrap so with every step you take some of the bubbles burst and just like bubble wrap whose bubbles have been burst you end up with an cushioned and flat support system where there was once cushioning.
The knock on effect of this cushioning breaking down is that the rest of the support and structure of your shoe also breaks down which can result in you not running in the same way as you first did when you were originally fitted for your trainers. This means that your feet, calves, quads, hamstrings and hips may not go through the same motion as they first did when you were first fitted for your trainers. The knock on effect of this is that the body will compensate for this mismatch and this compensation leads to not running as efficiently as you once were which can result in niggles or a long term injury.
As the breakdown of your trainers happens slowing, you don’t just wake up one morning and the trainers are no longer effective, it may be hard to pinpoint exactly when you need to replace them. Here are my top tips for recognising the signs of when to replace your trainers
You start to have niggles in places that you’ve not had niggles before, it may be that you are starting to have aches, pains or injuries that you have not had before
You have run over 500 miles (again this is difficult to determine as weather and terrain can also play a part in the deterioration of the trainer)
Even if you have had your trainers for a long time but you haven’t run the suggested miles in them, adverse heat and cold can break the trainers down if they have been thrown in a cupboard
The rubber on the outsole has worn down on one side more than the other
If your trainer bends in the middle easily (the support has therefore broken down)
There is little or no give when you press firmly on the inside of the front of the shoe