ING Miami Marathon and Half Marathon director David Scott answered his phone Thursday to discover a panicked runner.

“I can’t believe this! I have my hotel booked, I have my flight booked and I have my rental car, but I can’t seem to register as I have in the past,’’ the person complained.

Scott was just as incredulous.

“And you waited until three days before a marathon to register?’’ the race director replied.

This week, calls like that have increased to a point where Scott and fellow race organizers are asking themselves, “Is this really happening?’’

It is. And it has.

The ING Miami Marathon and Half Marathon, which will draw 25,000 runners at 6:15 a.m. Sunday in front of AmericanAirlines Arena, sold out this week for the first time just days before its 10th anniversary.

No one was quite sure if another marathon — after previous versions — would survive in a tropical setting that could potentially wilt the heartiest of athletes.

But this 26.2-mile race (about 6,000 registrants) and its 13.1-mile counterpart (about 19,000) have gotten more vibrant and apparently more vital with age.

Popular event

On Friday, runners crowded into the Miami Beach Convention Center to pick up their race packets and attend the Nissan Health and Fitness Expo — a potpourri of fitness-related booths, seminars and retail areas.

In the midst of it all, the ING event had its prerace news conference, complete with city and county officials, race organizers and elite competitors from around the world.

Scott was mostly worried about telling people who assumed they could register at the expo — which has usually been the case — that there were no more spots. The race posted on its website that it was reaching its 25,000 cap, and newspaper articles warned folks weeks ago but …

“This is new water we’re wading through,’’ said Scott, who then had to inform runners Friday that even the Publix Tropical 5K Saturday in Miami Beach had reached its limit of 2,500 participants.

He said the marathon needs “controlled growth. We want to be doing it right. We don’t want to change the symmetry of the event. It’s been manageable, and we’re not here for just this year. We’re here for longevity.’’

Rain possible

The scenic course winds through some of the prettiest and most eclectic neighborhoods of Miami and Miami Beach.

Last year’s conditions were nearly ideal, though this year’s forecast isn’t as promising. A 40 percent chance of rain was predicted, with showers and temperatures in the low-60s forecast for the start. Temperatures were expected to rise into the 70s, with continued rain, or at least high humidity.

The marathon, which has competitors from every state and 79 countries, will award prize money to the top three overall men and women — $2,000, $1,000 and $500, respectively.

The male and female wheelchair winners get $500 apiece in the hand-crank and push-rim divisions. The half-marathon winners earn $1,000.

Bonuses of $500 apiece go to course record-breakers.

Some story lines for Sunday:

• Alena Vinitskaya, 38, of Belarus, has returned to defend her 2011 title (2:44:38). Vinitskaya was ING runner-up in 2003 and 2010.

• Benazzouz Slimani, 37, of Morroco, the 2009 ING Miami Marathon winner in 2:16:49, and runner-up last year in 2:18:23, will go for his second title.

• Stacie Alboucrek, 44, the Fort Lauderdale resident who won the ING in 2004 (2:42:32) and finished second in 2011 (2:49:03), will compete again in the marathon.

Marathon favorites

Among the marathon favorites is Samuel Kipkosgei Malakwen, 33, of Kenya (2:11:31 best), who won Twin Cities last year in 2:23:12; and finished third at Grandma’s Marathon in Duluth in 2:12:39.

Ethiopian Ambesse Tolosa Wolde, 34, has a 2:08:56 best from the 2004 Paris Marathon; and Ethiopian Teferi Bacha Regasa, 25, won the 2011 Luxembourg Marathon in 2:15:43. His best, at the 2010 Milano Marathon in Italy, is 2:11:35.

The ING Miami’s 2011 men’s champion — Ethiopian Tesfaye Sendeku Alemayehu in 2:12:57 — will run the half marathon Sunday, as will Mitch Guirard, 26, of West Palm Beach. Guirard finished in 2:30:36 last year in his first marathon attempt. His half-marathon best is 1:09:44.

The men’s Miami Marathon record is 2:12:22, set by David Rutto of Kenya in the inaugural race in 2003. The women’s record is 2:34:11, set in 2006 by Hiromi Ominami of Japan.

The half-marathon course record (1:03:46) was set by American Brian Sell in 2008. Yolanda Fernandez of Colombia ran the women’s record of 1:16:01 the same year.