There are two different arches on the foot: 1) The long arch on the medial of the foot and 2) The front arch, built up by the metatarsal bones.

Both arches can drop through overexercise or fatigue. Very often it is the front arch that gives way, and it can cause considerable pain while running. Orthopedic insoles  can help you support both arches, but they are not so good for long distances. They are made of hard materials, that start rubbing and putting pressure at your feet.

The front arch

For the front arch it is advisable to keep your normal insoles and stick metatarsal-pillows on them. The pillows are soft and don´t create any rubbing. Or you can do like 3100 mile runner Rathin Boulton, who used a piece of bandage, rolled and taped it on the insole. The most important thing is to take the pressure away from the metatarsal bones - to do this you have to put the pillow before the metatarsal bones (towards the heel bone) and not directly on them.

The long arch

For the long arch you can get arch bridges, which come sometimes with the running shoes and enforce the insole. You can also get insoles that lift the long arch to various degrees. Depending on the length of the race, it might be good to keep a couple of pairs of different heights on hand if you know you have an issue.