Organisers of the Great North Run have described the deaths of four competitors as "deeply unfortunate".
The deaths occurred during Sunday's 13.1 mile run in Newcastle, in which 50,000 amateur runners took part.
A spokeswoman said: "For there to be four separate and unrelated fatalities is an exceptional and deeply unfortunate occurrence."
Race chairman Brendan Foster said: "At this time our only thoughts are with their friends and families."
Organisers defended their safety preparations after the names and ages of the deceased men were released on Monday.
The London Marathon, Britain's premier mass-participation long-distance road race, has recorded just five fatalities from heart problems among 430,000 runners in the last 20 years.
"There are medical facilities at every half-mile, a fleet of more than 16 ambulances all with defibrillator units, two paramedic bikes and two field hospitals at the finish," said a spokeswoman.
"The third facility is a major hospital at South Tyneside within 100 metres of the 10-mile point of the course, with dedicated services for the competitors."
Temperatures during Sunday's run reached 20C, about four degrees higher than the average for the area at that time of year.
Humidity was also high and there was little wind to bring relief to runners on the course.
But organisers say that there were sufficient fluids available to runners.
"In addition to supplies at the start and finish, there were five water stations on the course and three specially constructed mobile shower units," said the spokeswoman.
"As with any large scale participation event, the entry-form states that all participants are strongly advised to consult their doctor prior to undertaking any exercise programme.
"As in all previous years, participants received comprehensive pre-race information about training, consulting a doctor, medical points on the course and further day of the race advice."
A spokeswoman for South Tyneside coroner's office said post-mortem examinations were being carried out on the dead men this week.