That depends on the person because a lot of people do not find having flat feet as being painful at all while other people do find them very painful because as it can affect their whole leg such as their feet, ankles, lower leg, even their hip and lower backs.
Flat feet can also cause problems with the bones, connective tissues as well as muscles that can be found around and in their feet and flat feet can make the feet roll inwards a lot more than they are intended to (this is known as overpronation). A lot of people who have flat feet go through a lot ofshoes quickly as well as having a lot of leg related injuries.
If you are a runner who has flat feet and feels pain either during or after a run there are many things you could do to make your feet more comfortable and I would suggest that you begin by looking into the different types of insoles which can relieve the pain of your feet while or after you go for a run.
These insoles are specially designed to offer you different attributes depending on what makes you comfortable. These insoles slightly thicker in the arch area providing extra support for your feet as opposed to the one-size for everyone that you would get with the insoles inside of shoes. Some insoles can be very firm while others can be quite soft and you can use them in all your shoes, not just running shoes because they come in different sizes as well as some of them are used for different routines that you do during your day. The best insoles for flat feet can help with the alignment of your legs as well as to prevent knees and ankle injuries