Cause of Foot Blisters
Blisters on the feet are usually caused by friction, usually between skin and sock. Excessive moisture due to sweaty feet or wet conditions can also lead to blisters. Wearing runners that are too small or tied too tight can also cause blisters.
Prevention of Foot Blisters:
To prevent blisters altogether, make sure your runners fit properly. Your runners should be at least 1/2 a size bigger than your normal shoe size, since your feet swell when you run. You should have a little room in your toebox.
Look for running socks or socks made of synthetic fabrics (not cotton!) such as Teflon or CoolMax, which wick moisture away from your feet, preventing the sock from bunching up and causing blisters. Also, buy socks with no seams and a smooth surface. Some runners wear double-layer socks created to deter blisters. (1000m socks) The idea with those socks is that any friction occurs between the two sock layers instead of your skin and the sock.
You can also spread Bodyglide or Vaseline on problem areas.
But go easy: Too much and you'll be sliding around in your shoes.
Don't remove calluses with a razor or emery board. The callused skin serves as blister protection for your feet, so if it's removed, you'll be more at risk for blisters.
Use Compeed or similar blister plaster on the blistered area and if prone to blisters put a blister plaster on before the blister develops using it as a preventive measure. If you do this, just make sure the plaster is applied smoothly (no wrinkles) and not too tight.
Treatment of Foot Blisters:
If you already have a blister and it's not painful, just leave it alone, since the skin serves as protection. It will eventually break and the fluid will drain. If the blister is painful, then you should sterilise a needle in boiling water and once cool, carefully pierce the blister. Press the fluid out and use an antiseptic cream on it. Cover the area with a blister plaster to protect against infection and provide cushioning.
While running the marathon, if you develop a painful blister, stop at one of the first aid stations. They should be able to treat your blister and hopefully get you back in the race, running pain-free.