It''s right around mile 20 that the pain gets real. All the distance-math you''ve been doing over and over in your head collapses into a jumble of numbers, a heap of ruined, false comfort. It doesn''t matter that you''ve neatly divided the previous 20 miles into three 10-kilometer races or two ten-mile runs, four five-milers or even 20 one-mile jogs. It doesn''t matter that you know there''s a finish line, that it really does exist, down the road somewhere. No matter what, that mile marker still says 20 on it. There may have been some psychological sanctuary in the number-games. but when you''re running a marathon, all the arithmetic only adds up to one thing and that''s Agony.

The mind takes over even more than before as the body goes numb. Running is a very mental sport, but even so, some runners--serious, well-conditioned runners--will say that at 20 miles, they can''t feel their legs anymore. The brain detaches from the body and yet the body keeps moving forward, like a headless chicken, only deliberately, not frantically. Other runners will depart their minds altogether, begin babbling gibberish and become genuinely rubbery. Still others just can''t take it. They succumb. They walk.

Yet no matter how awful you feel at mile 20, there are still six miles and 385 yards to go that can only be traversed by placing one foot in front of the other. And for anyone running the Big Sur Marathon, being past mile 20 means the worst is yet to come.

Big Sur is more hateful than a normal marathon. The last leg of the Big Sur is six miles and six hills of twisty, banked road through Carmel Highlands. There is no mercy. Those miles are so dreaded for what they do to knees and hips that veterans will groan at the mere memory.

Then again, if you''re a runner, that''s the whole point. No joy is free. The rewards of running a marathon must be earned. And the reward is a feeling so sweet, it''s impossible to know without just doing it. Long distance runners get high from the miles they run, and from all reports, there''s no high like a Big Sur high.

Wally Kastner is the race director for the Big Sur International Marathon, which is scheduled this year for April 28. A retired Marine lieutenant colonel, he has run 40 marathons and racks up about five a year at age 56.

Kastner is fit. About the Big Sur race, he''s blunt.

"Runners will go through moments of severe pain on our course," he says.

Severe pain can be expected in any marathon, but in Big Sur there are a few factors besides the incredible natural beauty that set it apart. For one thing, it''s not a flat, urban course. Although there''s a net elevation loss between the starting point at Pfeiffer and the finish line in Carmel, the Big Sur coast undulates. A cross sectional map of the course reveals many hills as well as a two-mile, 500-foot climb at Hurricane Point which slides back down the other side to Bixby Bridge. Mercifully, race organizers have various musicians posted along the route and every year they place pianist Jonathan Lee at the bridge, in position to gently cradle weary runners as they descend.

"People say they start crying coming down the other side," Kastner says.

Kastner''s training advice for Big Sur is simple: "Hills."

In this area there is plenty of terrain to train on. Kastner often runs from downtown Monterey up to Jacks Peak and back. You can also drive to Jacks Peak, park right at the vista point and run up the hill from there.

"It''s the downhill that hurts you at Big Sur," he says. "You just tear up your legs."

But a factor more specific to running Big Sur is the native wind. If it''s blowing it means there''s no fog, and thus the coastal splendor is visible. Seeing the ocean is nice, but strong wind just makes the run harder.

"There''s a 35 percent chance of extreme headwind and 100 percent chance of wind," Kastner says. "You''re going to have wind out there somewhere."

One runner who''s done Big Sur three times calls the wind "super brutal." So that she wouldn''t bear the brunt, she tucked herself behind packs of runners when it got gusty. It can really blow hard. "One year you came around this corner and it really blew you off your feet," she says.

Natural obstacles aside, the natural beauty of the Big Sur marathon is what makes it tolerable. Dedicated runners often prefer to run in beautiful places rather than city streets because it keeps the mind occupied and each run fun. For that reason, as well as its excellent reputation, the Big Sur Marathon is ranked as one of the best in the country. The sign-up list was full back in October.

The inherent difficulty of the Big Sur course, combined with its beauty, the music and the fact that there are parts where a runner is totally alone on the edge of the continent, can make for a surreal and, some say, mystical experience.

Kastner says, "I won''t say weird. It''s different."

When it''s all over, finishers sit together to eat, drink beer and swap war stories. That is, if they''re able to stand.

"You can feel great or you can feel bad," Kastner says. "It all depends on how hard you work."

And for anyone planning to run the Big Sur Marathon, that means you should be working hard on those hills. Otherwise, those last six miles will be worse than the worst.