On the day a 22-year-old runner died in hospital after collapsing following the London Marathon, a BBC journalist who also fell ill shares his story.
Tom was released from hospital in London on Monday
BBC Wales online journalist, Tom Bourton, 29, tackled the marathon for the first time but ended up in hospital as one of the heat's many victims.
I really don't know what happened.
One minute I was running along enjoying the buzz of the crowds, the carnival atmosphere and feeling pretty good.
I had reached 17.5 miles and felt I had broken the back of it.
The next thing I know I am lying on the floor surrounded by St John ambulance workers.
Running a marathon has been a goal of mine for years and I have put in months of rigorous training and I was feeling pretty confident at the start line.
The concept of not finishing never even occurred to me, let alone the idea of being hospitalised with a steady stream of saline drips being fed into my arm.
During the race I had been taking on plenty of fluids but on such a hot day it would appear I did not take on enough.
For the first few moments lying at the roadside all I could think about was how it had really messed up my day.
The plan had been to celebrate the culmination of months of training with a well-earned beer. Now my first concern was causing distress to my wife and parents who were waiting at the finish line.
The idea of trying to continue did cross my mind, but Derek the paramedic said that, although he could not physically restrain me, he would do everything else in his power to stop me.
And quite frankly it would have been a very, very bad idea - which I probably knew all along but it just shows how desperate I was to finish.
Another though that flashed across my mind was the dreaded prospect of having to do another marathon and all the training that that entailed.
Tom pounded the streets of Cardiff in training for the marathon
Then there was the embarrassment of not finishing and the feeling of letting down all the people who had sponsored me nearly £3,000 for the NSPCC and wished me well.
I was also so bewildered that I even asked if I was in danger of dying.
It really felt like the lowest point of my life.
When I gained a bit more strength I started to lift my head and take in a bit more of my surroundings.
The main thing I could see was the runners flowing past, many of whom looked at me, waved and carried on - hopefully with a stark warning of the dangers of the day.
But as I looked more closely I could see many more fallen victims of the heat strewn across the pavements. Callous though it sounds it made me feel better that I wasn't the only one.
'Took on liquid'
One man who was lain down next to me was in a far worse state and he was barely conscious.
And when an ambulance managed to get through to us he was taken off first.
Shortly afterwards I was wheeled to the nearest first aid point through the runners which was not a pleasant ride.
At the tent I lay down and waited. I was still confused and when my mobile phone I rang I could not remember how to answer it even though I use it every day.
My wife and parents eventually made it to the tent just before my ambulance arrived and I was taken off.
Scores collapsed in the heat during Sunday's race
At first we were told I was going to the Royal London, but on the way we learned they were full with marathon runners, so we were diverted to Newham University Hospital which caused more fun and games for my dedicated followers.
In there the A&E was "marathontastic" too. Most with very similar stories to mine - good training, took on liquid, but still ended up in A&E.
After more drips, ECGs, blood pressure tests, urine samples and blood tests I was told they wanted to keep me in overnight which was another blow.
On the plus side one of the doctors who examined me said that he had not tested someone who was fit and well since medical college, so it all seemed precautionary and laid to rest my fears of the collapse being a symptom of a larger medical condition.
As I lie in hospital - the first time I have ever had to stay overnight - I am torn between hanging up my running shoes for good, or tackling another marathon to banish the demons.
It is a decision I won't be making in a hurry.
Tom would like to thank all the St John ambulance staff who helped him and the staff at Newham hospital. He was released from hospital on Monday afternoon.