By Richard Black
BBC science correspondent
Some Eureka moments lead to really good ideas. Others produce really bad ones. Last November, I had what could be one of my worst ever.
Richard Black tried to find out what happens to your body when you train
It was on a sofa-bound evening, remote control in one hand and glass of Chardonnay in the other, that the idea of running the London Marathon took hold.
The dénouement is now frighteningly close at hand; to be precise, this Sunday. But this hasn't just been a personal quest.
As I considered what it would take to transform the droopy wine-bag around my middle into a finely-honed six-pack, journalistic instincts began to kick in.
For example, what happens to your body when you train? How do you know what to do for the benefit of muscle, blood vessels, joints, and the psyche?
Fortunately BBC World Service agreed this was fertile ground for a radio series - Marathon Man - hence allowing me to get some of the best advice around.
I received words of wisdom from the current world record holder Paul Tergat - now sadly ruled out of the London race with a calf strain - and from experts in the fields of physiology, nutrition, physiotherapy, and motivation.
"The marathon is not like running 5km or 10km or a half-marathon," Paul Tergat told me at a restaurant in Nairobi, where I was relieved to see him tucking into a big plate of meat and vegetables with relish.
"The marathon demands proper preparation, it needs a lot of detailed preparation; and the key thing is that you should take at least six months in getting ready so as to avoid a lot of problems in your body.
"The next day you will end up with sore muscles. I'm a professional marathon runner now, but I still find the next day it's hard to climb upstairs because of my sore legs."
Tergat told me that he runs around 240km each week in training - which if you do the maths equates to very nearly a marathon distance (though not at top pace) every day!
But for slobs like me, the secrets of effective training are distance and variety, according to marathon trainer Vickie Aldridge.
"If you can get to 70 or 80km per week, that would be great," she told me.
"Each week you should do a long, slow run; a tempo run - at the pace you will use in the race - some hill work; and maybe a speed run; so you will be doing something pretty much every day."
Like many people who set off on the marathon quest - and over half a million globally do so each year now - my main goal was more fitness, less fatness.
So I wanted initially to see how fat I was and how fit I was not.
At the Human Performance Centre in London's South Bank University, Vassiliki Costarelli sought out my flab.
Calculating percentage body fat involves using a set of callipers to measure the thickness of skin folds at four standard points on the body - bicep, tricep, subscapula (just under the shoulder-blade) and suprailiac (on the front corner of the hip).
Muscle genes change their patterns of activity
These four measurements are plugged into a mathematical formula to work out percentage body fat.
"Unfortunately it's not that encouraging," was Dr Vassiliki's verdict.
"The callipers have shown 21.8% body fat; now that's absolutely fine for a female (women naturally have a higher percentage fat than men) but not for you."
She did have the grace to be surprised, however - as I was - when she found that according to another measure of fatness - body mass index - I was obese.
Body mass index or BMI is, quite simply, a ratio - weight in kilograms divided by height in metres squared; more than 30 is defined as obese, between 25 and 30 as overweight.
Puddle of sweat
Mine came out as just over 30. Oh dear. It looked like I'd be joining Paul Tergat on the kilometres but not on the meat and veg.
The first kilometres of many would be run on the Centre's treadmill under the supervision of Steve Hunter.
Steve was going to assess VO2max - basically the maximum amount of oxygen which the body is able to take in through the lungs and use during exercise.
Looking like a fetishist's dream in rubber facemask and adhesive chest sensors, I climbed on the treadmill with some trepidation.
"We're going to get you on the treadmill and we're going to slowly increase the speed or the incline until we reach your point of exhaustion, which should mark your maximal oxygen uptake," he said with what could have been mistaken for a sadistic glint.
"As you breathe out, we take a sample of the air through the facemask, and that goes down into the machine, which samples that expired air for concentrations of oxygen and carbon dioxide; and from that we can get the measurements of oxygen uptake and carbon dioxide output that we need to evaluate your fitness."
The chest sensors meanwhile were to measure my heart rate. A person's maximum heart rate can be roughly estimated by subtracting their age from 220. So at the grand old age of 42, mine should be around 180.
Twenty minutes later, after he'd turned me into little more than a puddle of sweat, Steve showed me a graph plotting oxygen uptake against time; and towards the end of the session, when I was working as hard as I could, lay this magic figure of VO2max.
"It gives us a result of 44.2ml of oxygen per kg bodyweight, which is actually quite good," he told me.
"If we looked at an elite athlete, they might be somewhere around 75 or 80; so there is some room for improvement."
Another figure Steve produced is called RQ - the ratio between carbon dioxide produced and oxygen used.
During normal life we burn fuel - glucose and fatty acids - aerobically. Each molecule of glucose combines with six molecules of oxygen to produce six molecules of carbon dioxide - so the RQ of glucose-burning aerobic exercise is one.
But when the body is under stress - as mine certainly was on Steve's treadmill - muscles start working anaerobically - converting glucose to energy in a reaction which doesn't use inhaled oxygen, but which does produce carbon dioxide.
So an RQ greater than 1 - as mine was - shows that muscles are starting to work anaerobically, which leads to a build-up of the infamous lactic acid, making you - to use the scientific term - knackered.
"So when you're training, you should try to keep yourself just below that threshold - and the same thing when you're actually running the marathon," was Dr Steve's advice.
And that's where heart rate comes in.
"We can see now where that threshold appears in terms of your heart rate so we can give you a very objective measure; if you train at that threshold, you're stressing the cardiovascular system to pump enough blood round to the muscles to prevent that transition to anaerobic exercise from occurring."
So I now knew how to exercise - but how to eat could be just as important.
The idea is to build up stores of a compound called glycogen.
"Glycogen is the substance in which the body stores glucose - it doesn't store it as free molecules of glucose, which is what the body actually burns," says Dr Mike Stroud, of the University of Southampton, a physiologist who last year, together with Sir Ranulph Fiennes, ran seven marathons in seven consecutive days on seven continents.
The body's tissues respond to a 'change in their environment'
"We have a store of glycogen in our muscles and another in our liver."
There is plenty of good advice around on eating for marathons, so I won't go into it here; suffice it to say that I've always loved bananas, so it hasn't been a problem.
What I was more interested in was how exercise changes the body.
"Your heart gets stronger, it can on every beat pump considerably more blood," says Mike Stroud.
"In your muscles, the fibres are getting bigger and stronger and so can contribute more; and the enzyme systems that allow the oxygen to be combined with fuel can do the job for longer at higher rates.
"And combined with all this, the small blood vessels, the capillaries, which go in and around all of these fibres to bring in what's needed and take away the waste, become much richer; so you've got a much better supply, and you can go on for much longer at much higher intensities."
At the Karolinska Institute in Stockholm, Carl Sundberg is trying to discover the genetic mechanisms which underlie these bodily changes.
"When one runs or walks or cycles or whatever, the tissues that are involved - that is the muscles, the cardiac muscles, even the brain - they read what is really a change in their environment," he says.
"And genes respond by altering their activity either upwards or downwards."
Dr Sundberg's team has just finished an analysis of how leg muscles respond genetically to exercise.
"Out of tens of thousand of genes which we looked at, we found 140 which are changed in their expression by exercise - they are either up-regulated or down-regulated.
"Some of these changes are related to the growth of new blood vessels; they drive the growth, and some change the local environment making it possible for them to grow."
Dr Sundberg is one of the early pace-setters in this particular scientific race. He acknowledges that the starting pistol has only just sounded, but believes it's possible in principle to discover genetic changes which mediate all the bodily consequences of exercise - even those which make us happier.
As marathon day approaches, I could use some of those happy changes now.
Even after some of the best advice in the business, 42km seems a very long way.
I will carry the wisdom of all these experts with me - but more than any of them, the encouragement of a world record holder.
"Running is a very simple thing," says Paul Tergat. "Once you have shorts, a pair of running shoes, you are off - you can become a champion."