Amid growing concerns about high temperatures on race day, the Boston Athletic Association announced a deferment option for this year’s Boston Marathon and plans to keep the course open an extra hour. Many runners learned of the changes from updates posted at the number pick-up area at the John Hancock Sports & Fitness Expo Saturday morning.

The latest forecast for Monday calls for temperatures to be in the 70s at the start, and reaching 85-90 degrees during the day.

Because of the unseasonably warm weather, the BAA will defer the entry of entrants to 2013 if they decide not to race. This includes everyone from qualifiers to charity runners.

To receive the deferment, entrants must claim their bib packet at the expo before it closes Sunday evening but not start the race. The BAA will follow up with next steps.

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

Said race director Dave McGillivray, “This gives them a choice. They have to assess for themselves if they can handle these conditions.’’

By keeping the finish systems open an extra hour - until approximately 6 p.m. - organizers hope to encourage runners to take it slow, if needed.

“It allows for an additional two minutes per mile to slow down and be sensible about it,’’ said Grilk. “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.’’

McGillivray said he thought about an earlier start, but that it would be virtually impossible to make it work, considering the organization of eight towns, the transportation of athletes, and other details.