"The blood apparently clears of mercury in the first few months post amalgam removal (If I recall correctly) because it's no longer getting the daily intake of mercury - but then later the stored mercury begins to leak back out into the system."

Not exactly. The blood level of mercury rises substantially by the removal, but in around a month it is back to the pre removal amount, then it drops from there. It won't drop that low though until most the stored mercury in the body is removed by many rounds of chelation. Taking large doses of vitamin c after the dental work will help clear the novacaine from the body. I often felt nausious after dental work. I don't know if it was from the novacaine, or the mercury exposure. It was just for around 2 days. Taking 5 grams of vitamin c a day for a few days helps(space it out throughout the day. You could try taking more vitamin c than that, but if you take too much it will give you diahrea). Taking B vitamins is also important for detoxification. Get a bottle of B50 tablets. breat them in half and take it 3 or 4 times a day.

If you can register for the run at the last minute, then see how you feel on the day of the run and make your decision to run or not then. It also depends on how many amalgams are replaced. If there are 6 or less it might not be so bad. If there are way over 8 of them, then you might not feel so well for many days after replacement.