An 18.5-mile open streets outdoor biking and half-marathon event planned for Sunday in the San Gabriel Valley may be postponed or shortened because of unhealthy air quality and logistical problems stemming from the San Gabriel Complex fire, organizers said Wednesday.

Organizers of 626 Golden Streets, billed as the largest ciclovia-type event in California history, launched into “contingency planning” Wednesday, the third straight day of a smoke advisory from the South Coast Air Quality Management District for the valley and Inland Empire calling on everyone to avoid vigorous outdoor activities and those with respiratory illnesses to stay indoors.

Two brush fires, one in San Gabriel Canyon near Azusa and the other in the hillsides of Duarte, continued to burn Wednesday in the Angeles National Forest as the San Gabriel Complex fire, charring thousands of acres, sending billowing smoke and gray ash into the region stretching hundreds of miles.

If air quality remains tainted, or the fire spreads, organizers may have to cancel the event, a major blow to bicycle, transit and city groups planning the bike ride for two years. From 8 a.m. to 2 p.m. Sunday, parts of Mission Street, Garfield Avenue, Huntington Drive, Colorado Boulevard, Duarte Road and Foothill Boulevard will be open only to bicyclists, walkers, joggers, skaters and others riding nonmotorized conveyances. The jagged, east-west route hugs the Gold Line light-rail from South Pasadena through San Marino, Arcadia, Monrovia, Duarte and Irwindale to Azusa, connecting seven foothill cities and six train stations.

Short of postponement, another option would be to truncate the route by excluding Monrovia, Duarte, Irwindale and Azusa, cities most affected by the twin fires, organizers said Wednesday.

“They are not ideal conditions to host a public open streets event,” conceded Wesley Reutimann, executive director of Bike San Gabriel Valley, the main organizer. Reutimann and South Pasadena City Councilman Michael Cacciotti, co-organizer, said they will meet with the South Coast Air Quality Management District, Los Angeles County Fire Department and various cities’ officials in the next two days and announce a decision Friday morning.

Reutimann and Cacciotti both received calls from Duarte Mayor Sam Kang, who said he was concerned about hosting an outdoor bike ride through his city. Cacciotti, a member of the AQMD governing board, said he has received calls from other elected officials and would meet with AQMD Acting Executive Officer Wayne Nastri on Wednesday or Thursday.

The AQMD reported Wednesday the smoke is moving eastward, away from the 626 Golden Streets route. Cacciotti said all indications were positive and he was optimistic the event would be held as planned. But the fire was only 10 percent contained Wednesday afternoon, and fire personnel said it’s still pumping out a tremendous amount of smoke.

“All of us want to go forward,” Cacciotti said. “It looks like the fire is getting better, and that’s in our favor. But of course, if there are big public safety issues we wouldn’t.”

Another problem is the temporary closure of streets around Duarte, Monrovia and Irwindale, which could block the movement of fire engines and other emergency vehicles from the staging grounds at Santa Fe Dam to the fire. One pinch point on Sunday would be Foothill Boulevard between Irwindale Avenue in Irwindale and Encanto Parkway in Duarte, scheduled for closure for bike riders, Reutimann said.

Also, officials from Monrovia and Duarte said public safety personnel guarding evacuated neighborhoods and busy with fire-related street closures may have their hands full. If the situation remains the same or worsens Sunday, they would not be available to direct traffic during the event. If more staff were needed, these cities would have to contract out, something not covered by grant monies allotted for the event, Reutimann said.

He noted the irony in the fact that a car plunged off Highway 39 near Morris Dam on Monday and burst into flames, causing the first fire that burned 1,200 acres.

“Hard to believe that two years of planning may literally go up in smoke as a result of an auto fire,” Reutimann wrote in an email.