excrutiating knee pain after 20mile run

I have been training for my first FLM last week when i ran 16miles i was pleased with the way it went but for the last couple of miles my knees were incredibly painful on each impact. It was recommended that i took Glucosamine and cod liver oil, which i did. On Tuesday i ran 20 miles and this time the last 4miles bought tears to my eyes :cry: . I am now going to invest in sorbothane insoles but am still fearfull of the pain i will feel on the day.
Can i take pain killers during the marathon?
Is there anything else i can do to ease the pain?
Would knee supports help?
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