To run or not to run?

Im kind of wondering if I should do the marathon this Friday. I picked up an Achilles strain on Monday. Its not really causing me any discomfort or anything, but a marathon can turn a simple strain into something that will knock you out of running for six months. A doctor friend of mine recommended anti-inflammatories such as Ibuprofen, but I got another piece of advice that painkillers probably aren't the way to go because youre relying on the pain to tell you whether to stop. I reckon I'll just turn up at the starting line and see what happens.