The validity of this account has been widely disputed among historians due to differing narratives from various sources. Some records mention a runner named Philippides (instead of Pheidippides). Others claim that the whole army marched rapidly to Athens, not just one messenger.
Fortunately, speculations over the details of the first marathon can now be quelled. Pheidippides' journals have been discovered and, by wild chance, have fallen into my hands. I'm going to paraphrase his personal account below. I would do more than paraphrase, but it's all written in Greek, which I don't understand per se. My interpretation is based mostly on the look and feel of his handwriting.
About a year before the Battle of Marathon, Pheidippides was 60 pounds overweight. "I'm pathetic", he said to himself as he stared at the scale one morning. "How could I have let myself get this fat?"
He resolved to get more fit, and decided the best way to do it would be to get a side job as a messenger. He started off easy - only delivering messages that were a mile or so from his home, but gradually he worked up to longer distances. His sister, who had been a long time messenger, invited him to deliver a couple of messages that were 5 and 10 kilometers away, and although he was a bit slow, he delivered the messages successfully.
After approximately six months of delivering messages, Pheidippides was feeling pretty good about himself. He had lost his excess weight, and was able to deliver several 5 to 10K messages each week. He decided to set his sights on something bigger. He had heard about the big battle of Marathon that was going to take place in half a year, and on a whim signed up to deliver the victory message from Marathon to Athens. After all, everyone knew the Greeks would win - it was really just an exhibition battle. (The Persians were a bunch of wimps.)
Some claimed that Pheidippides was a great patriot for voluteering to deliver the message, but really he just wanted to get the "κϛ.β" sticker for his chariot.
Pheidippides' sister agreed to deliver the message with him, and together they began to prepare themselves physically for the mighty task.
Pheidippides' wife was supportive, but a little concerned. "Are you sure this is a good time to deliver a message over such a long distance?" She pointed out that he was very busy with his full time job, and that he had recently joined the cathedral choir - which demanded a lot of time.
Pheidippides waved away his wife's worries and assured her that preparing to deliver the big message would not impact his life significantly. Pheidippides was wrong.
The ensuing months proved to be very difficult. With the hours of training added to his already busy schedule, Pheidippides began to think he had spread himself too thin. Still, he pressed on. He met his sister each Saturday to deliver messages and the distances grew longer and longer.
At long last, the day of the battle approached. Pheidippides and his sister travelled to the battle site with another messenger friend the evening before the victory was to be won. That night they made camp, but slept very little due to the fact that their camp site was right next to a umm... train station.
They awoke very early and got on one of the many chariots designated to transport all of the messengers to the place from which they would depart. Upon reaching the starting point, they huddled in the brisk morning with thousands of other messengers (apparently the Greeks believed in redundancy) and awaited the explosion that would announce the end of the battle and the beginning of their journey. They heard the blast and began.
The first several miles passed in relative ease. It was a beautiful morning. The sun was shining, the cool mountain air was refreshing and there was an electricity in the air that was exhilarating. Some messengers were faster than Pheidippides and his sister. Some were slower, but no one cared. They were all just happy to be there delivering such an exciting message.
When they reached the half-way point, Pheidippides was pleased to see that, according to his GPS enabled wrist mounted sundial, he and his sister had traversed the distance much more quickly than they had expected. He was tired, but still felt strong and confident. They pressed on.
As the day progressed, the sun burned away the crisp air of morning and strength began to dwindle. The road stretched endlessly before him, and Pheidippides noticed that the distance between mile markers was growing. He plodded arduously for what seemed like hours only to find seconds had passed. At mile 17 he had a fleeting fear that he might not be able to deliver the message after all. He pushed the thought out of his mind and trudged on.
He passed mile 20 with a feeling of accomplishment. Never in his life had he delivered a message farther than 20 miles. Every step he took now was a new best for him.
Pheidippides had heard about "the wall" that messengers encountered when delivering messages over long distances. At mile 22 he began to wonder if he had already hit this wall. He was tired. His muscles screamed and his joints ached. Maybe this was all "the wall" was... It wasn't. At mile 24 Pheidippides learned what "the wall" was. His body felt like it shut down. Every single step was a battle of will. He wondered what in the hell ever possessed him to deliver this stupid message.
On and on he struggled. The path opened to a large clearing and Pheidippides was distantly aware of thousands of people cheering all of the messengers on their way. He could see his destination far down the road and it may as well have been on the moon. Every part of him wanted to stop. His hands, feet and head felt as if he had sat on them too long and they had gone to sleep. His vision was a blur and all sound seemed to be filtered through a pool of water. Still he plodded on.. and on.. and on... until finally he stumbled weekly through the entrance to the assembly.
Relief and exhaustion flooded over him. He shouted triumphantly, "WE HAVE WON!"
Then he died.