While Scotland Yard is co-ordinating a major security operation to limit the chances of a bomb attack, marathon officials believe the elements pose a greater risk.
Race organisers urged the public to be vigilant, but said they were more fearful that the event would be disrupted by British Gas or Thames Water, than the IRA.
Contingency plans have been drawn up to guard against the possibility of collapsed roads, burst gas mains and flooding.
Yesterday the Cancer Research Campaign created a further scare when it announced that runners were at risk from skin cancer if they failed to protect themselves against the spring sunshine.
Nevertheless more than 500,000 people are expected to turn out to watch a field of 28,000 runners.
The bomb threats which led to the postponement of last weekend's Grand National have heightened fears that the marathon could be the next event on the terrorist hit list.
When the National was finally run on Monday, racegoers were told not to bring baggage to the Aintree course and were searched at the turnstiles. Such thorough checking would not be possible at the marathon.
With more than 26 miles of public highway to monitor, the race is the most difficult major sporting event to protect from such attack.
Nick Bitel, the marathon's chief executive, said that during its 17-year history the race had learned to live with the threat of terrorism.
He said: "Some years ago, two days before the marathon, there was a bomb at the Baltic Exchange which is virtually on our route. More recently, the first vehicle to go down Marsh Wall after the bombing there was ours, putting down the blue line for the marathon.
"But it is not just a terrorist threat that could take out one of our roads. Water mains go down all the time and if a hole appears in a road you have a major problem." Three years ago a burst gas main at a key point on the course led to major difficulties.
This year race organisers are concerned about the commercial threat of "ambush advertising". Companies which are not sponsoring the race have sought to cash in on its popularity by placing prominent advertisements along the route. There are fears that the practice, which is legal in Britain, could lead to loss of sponsors at future marathons and other sporting events in this country.
Even the glorious weather was yesterday denounced as a threat to runners and spectators. Professor Gordon McVie, director general of the Cancer Research Campaign, said: "Skin cancer is the fastest growing cancer in the UK and because it's only April, many people wrongly believe they are not at risk from the sun.
"Marathon runners and spectators are particularly vulnerable because many of them will be out for most of the day and they won't be covered up."
The charity, which is being sponsored by 387 runners in this year's event, said that, where possible, people should seek natural shade, wear cover- up clothing, avoid the midday sun, and use a sunscreen with a protection factor of 15 or above.