What have I got myself into?

So. Marathon training.

The clue's in the name.

When I decided to run the London Marathon I didn't give much thought to the process of preparation. Sure, I thought it would be pretty hard: you don't just get out of bed one day and run 26 miles without some kind of limbering-up process. (Well, you can, and people do; but they're usually the ones on a stretcher or on the ground somewhere around mile twelve.)

Way back in June I asked my friend Kate -- who has run the marathon in a time that makes grown men blanch -- what the training involved and just how tough it would be. She laughed and said: "You just run. And run. Then you keep running. And you eat lots of bananas."

So I started running. Somewhere in the early months -- back in summer, those lovely days of sunlight that I took so much for granted -- I realised that getting a marathon place wasn't as straightforward as I had expected. Naively, I had imagined some kind of scenario in which I would step forward proudly, perhaps slightly bashfully ("No, really, don't clap") and giving a gracious nod, announce my participation, upon which grateful organisers would deferentially usher me in, perhaps shaking my hand and patting my back.

In reality I had to put my name into a ballot, pay a fee  -- refundable, but only if I was too much of a miser to donate it to charity -- and hope to beat the odds to be one of the lucky ones. Everyone I told regaled me with tales of people who had applied for years with no success.

I then realised (it's all in the research, folks: aren't you glad I've done yours for you?) that if I wanted to run the marathon for the National Autistic Society, I needed to apply to them separately. So again I stepped up, filled in the form, and waited to hear trumpets. Thank you very much, they told me, we'll be making our decision in a couple of months and we'll get back to you.

So I ran the Royal Parks half-marathon to feel like I was doing something while I waited: to get strong for Grace, to tell people what her life is like, to run away the stress, to secure that place on the NAS marathon team and to prepare for the biggest physical challenge of my life.

And here it is.

What they don't tell you is that marathon training is basically running in the dark. (Unless you don't work and can fit it in during the day, in which case you have it easy my friend: go and look for running sympathy elsewhere.)

There are several options around running in the dark. You can run in the dark in the morning, before work, when the streets are cold and silent. You can run in the dark in the evening, once work is over and the kids are fed and bathed and put to bed, when the streets are cold and silent and every window you pass frames an imagined scene of cosiness and languor. You can also pretend you're not running in the dark -- there are a couple of options here. You can run on a treadmill with the light on, forcing wakeful cheer as you grind away on the spot in the spare room and outside the streets are cold and silent apart from slightly hysterical marathon runners who are at least breathing fresh air and watching passing scenery rather than that unloved armchair and the clothes rack. Or you can run outside with your eyes shut. This one doesn't work.

The exception to running in the dark is the weekend, when you get to stride out under brighter skies (or grey skies, or rainy skies, or windy ones.) This is bliss, not least because running in daylight means parks and woods are safe again and you can pace to the sound of leaves crunching or birds singing: even in inclement weather the drip of rain on trees and grass is infinitely preferable to the dreary patter of it on pavements. The downside of this is that it's your Long Run (this is the only run that other runners want to know about: "How far did you go for your Long Run?" "Long Run this weekend? How did it go?") So after a while running in daylight seems like less of a gift when it means you have to keep going for 10, 11, 12 miles. In the New Year these runs will reach 18, 19, 20 and I will have to set aside three hours or more and take a packed lunch.

But here's the thing. It's very, very hard, and it's daunting, and quite often these days I have a feeling in my stomach close to fear or panic when I consider what I've got myself into (six million television viewers in the UK alone are enough to give you stage fright, even if you are going to be running alongside 34,999 other people.) But then I will go out for a training run, and out of nowhere, magic happens.

It might be that I remember the money I raise will go towards giving worried parents advice via the Autism Helpline, or pay for a friend to regularly meet someone who has autism, or give practical support to someone with autism who is looking for work.

It might be that while I'm slogging along painfully I remember that Grace often feels the same way as she struggles to get to grips with school work, or understand the social cues of her friends, and that she doesn't get a day off, or the option to stay at home and eat chocolate on the sofa.

Or it might just be the huge adrenaline rush that kicks in sometimes and makes me laugh aloud with joy as I sprint and remember that the last anti-depressant I took was six months ago.

So yes, marathon running is excruciating, and monotonous and humbling.

But it's also the best thing that's ever happened to me. So if you'll excuse me, I've got to go and eat some bananas.