Susannah,
It is normally a good idea to break shoes in whilst you still have another pair that are usable. The trick is to overlap i.e. buy a new pair before the old ones are dead.
However, if the new pair are OK and they are all you have then great. Don't keep your mileage down but if you have another pair of (worn-in) shoes it may be an idea to split the mileage between pairs.
The problem comes if a new shoe irritates you anywhere or, worse, injures you. If they are all you have it can be tempting to keep on with them.
Also, it can sometimes take a while to adapt to new shoes if they have different cushioning or stability. It's why when you find a style that really suits it's best to stick with them.
Hope this helps/makes sense!