'Even Sir Mo Farah struggled' - The immense challenge of this year's London Marathon

This year's London Marathon runners battled sweltering conditions as the race recorded its hottest ever temperatures.

Here, Sky News correspondent Enda Brady describes the physical toll of the event which once dealt him "the worst physical experience" of his life.

Running any marathon is a massive test of endurance but when it gets as hot as it did in London on Sunday, the physical challenge simply becomes immense.

After months of training during the winter, runners were confronted with record temperatures and many were left cursing the weather gods for their terrible timing.

The organisers had been sending emails and texts to participants for days warning about the heat and the need to plan accordingly.

They laid on extra shower stations and more water and urged people to have a re-think if they were planning on running in fancy dress costumes.

My training partner Liz and I duly made a pact that we wouldn't go for PBs (personal bests).

Given the heat, we felt it would be an achievement in itself just to get round safely.

Down at the start area in Greenwich it was warm by 9.15am. You could just tell it was going to be a long, long day for everyone.

I collapsed with heat exhaustion at the 2013 London Marathon at mile 17 and it was the single worst physical experience of my life.

My body felt like a car that was running on fumes and about to conk out any second. And it did. Painfully so.

It was something I have never forgotten and I felt heartbroken hearing the news that a 29-year-old runner has lost his life.

Like everyone else, he will have trained hard, in cold weather, in rain, snow and ice. And then out of nowhere comes the hottest London Marathon in history.

And he will have pushed himself because of the generosity and support of family and friends sponsoring him and wishing him well.

The London Marathon is the greatest 26.2 mile race in the world, but it's a punishing one.

And in that kind of heat, every single finisher should look back on it and feel proud. And grateful.

At one point in the race we saw Sir Mo Farah - and even he was struggling.

If a champion like him was having it tough, that surely was a signal for us to slow down and just get through.