I fought that last year. My colt stepped on I think a nail. never really saw what. He abcessed, we did X-rays, we did antibotic drips in the vein in his pastern. I really struggled to clear it up.

After several weeks of working with my vet to heal it, and it did not heal. My farrier suggested a poultice made from 2 cups of salt and 2 egg yokes.  Mix it together and packed it into the frog after soaking it in Epsom salts. Duct taped the hoof up and left the poultrice as long as the duck tape held.

Maybe the Poultrice worked, Maybe the antibotics from the vet finally caught up with the infection. But it cleared up.  But it was a six week process.

Good luck

An abscess takes longer than two days, but I've never known an endurance horse and I'm sure those extremes change some circumstances.  Chances are the abscess was brewing and once he stepped on the nail he shifted his weight on that hoof and exacerbated the condition.  Thus bringing it to a head sooner rather than later.

I second the soaking in Epsom salts and using icthamol on the puncture and abscess.  I'd also have my farrier look at the hoof to see if he can speed up the healing process with corrective trimming/shoeing.

That little crescent shaped area on the heel may just be the place where the abcess broke through and not actually the horse stepping on himself.  Oddly enough, my horse (who I plan to do our first endurance ride in Dec ) came up with the same thing...He doesn't tend to step on himself either and was not lame but sometimes they don't go lame from abcesses.  The bell boots will definitely help protect him just in case and that way nothing else will rub against it until it is healed as well. 

Happy trails, Tammy

About 15 yrs. ago I did a 100 mile endur. ride w/ my mare.. she finished happily in 9th place, strong and sound, 2 days later was 3 legged lame.  Naturally I was frantic!  turned out she had an abscess- who knows what from and how long it was brewing.  I soaked her hoof in epson salts twice daily then immed. afterward wrapped w/ icthamol satutated gauze  for several days then switched to "sugardine" (betadine solution mixed w/ sugar) diapers work well to use for part of your wrap.  then put duck tape around the whole thing.  it took about 3 weeks to resolve. 

abscess (s) can be scary things.  sometimes they are minor and will just pop out the cor. band on their own but I've also seen them get infected and work their way into the bones.  that happened on a client's horse and it eventually had to be euthanized.  very sad...