View Photo Gallery: Runners in the Boston Marathon set out from the starting line in Hopkinton.

Updated at 11:28 a.m., with race underway

The forecast calls for such unusually hot weather for the 116th Boston Marathon today that the Boston Athletic Association offered runners the chance to sit this one out.

With temperatures in the 85-90 range expected during the day, the BAA emailed the race’s 26,716 entrants Sunday to remind themof the risks of running in a “red-zone condition” and to give them the opportunity to defer entry until the 2013 race. (Temperatures are not expected to be a problem for elite-level runners.) A little over 4,000 runners heeded the warning, according to the Boston Globe, with 22,426 starting the race.

“For many people, running the Boston Marathon is a once-in-a-lifetime experience,’’ said BAA executive director Tom Grilk told “We don’t want people to feel they have to run, because perhaps it’s not the wisest decision under these conditions.”

The BAA’s email recommended “... that anyone entered in the marathon who has not met the qualifying standards for their age and gender strongly consider not running, and that they strongly consider deferring until next year. Another essential factor to take into consideration is whether you have ever run a full marathon in weather conditions involving hot temperatures — and that can mean temperatures even lower than those that may be present on Monday. Do NOT assume that any experience you have in running a cooler marathon will be a reliable guide in making the decision in whether to participate or defer.”

Race officials considered starting the race earlier, but Race Director Dave McGillivray said that logistics — the race spans eight towns — made that impossible on short notice. Instead, the finish line will remain open an extra hour, until 6 p.m.

“It allows for an additional two minutes per mile to slow down and be sensible about it,’’ Grilk said. “Be part of what could be a well-remembered experience, but do it in a way that is careful. Ultimately, it is an individual sport and individual decision, but we want to make sure we provide people with the comfort of knowing they can make the decision.’’

The first competitors set off at 9 a.m., with the start of the race for the mobility impaired. The elite women go at 9:32 a.m., the men at 10. The final wave of runners starts at 10:40 a.m. Last year’s winners, Kenyans Geoffrey Mutai and Caroline Kilel, are competing in today’s race.

Capital Weather Gang: Forecasting the marathon