I've told quite a few people I'm running a half marathon, and one response has been particularly common: "Really?", swiftly followed by "Why?" (often followed by raucous disbelieving laughter, but we'll skim over that part.) I have set out to answer these two deceptively simple questions in the hopes of convincing you to make a donation to my cause.

"Really?" Yes. I am actually going to do this. I have never been much of a sportswoman. I was not half bad at the high jump at school, but I think that was mostly because it was a sport that required little endurance or training, relying almost completely on the fact that I was quite tall. It is an understatement to say that running has not really been my forte, and before this week I had never run more than 3 miles in my entire life. But I'm determined to give this my very best shot.

"Why?" Good question, and one I'd have asked myself a couple of months ago before I decided to take up this challenge. Well, there are three main reasons:

1. I watched Run Fatboy Run on Channel 4 one evening and I found it an inspirational piece of filmmaking about one man's fight against his own laziness. This is not an invitation for anyone to chase me around whilst I'm training and hit me with a spatula. Unless I ask you to.

2. My stupid competitive instinct meant that once I heard a few friends were doing the race, I was cross that I couldn't, so decided to try.

3. I really want legs like Karlie Kloss.

Jokes aside (although these reasons are all true), there is a much more important reason why I'm doing this to myself. I'm trying to raise money for an important charity that does vital work, Alzheimer's Society, the leading support and research charity for sufferers of all forms of dementia. I, alongside my family, have had personal experience of the impact this condition can have not only on the person suffering from it, but also on their carers. Degenerative brain disorders are heartbreaking, as anyone who has seen a loved one live with one will know, but with the right care and support they can be made much more bearable. Alzheimer's Society does fantastic work in helping to improve the quality of life of those affected by dementia, through practical local community support, and providing advice and information to sufferers and carers, as well as groundbreaking scientific research.

Any donations, whether large or small, will make all the difference and will spur me on at times when I hit the wall. Thank you.

Donating through JustGiving is simple, fast and totally secure. Your details are safe with JustGiving – they’ll never sell them on or send unwanted emails. Once you donate, they’ll send your money directly to the charity. So it’s the most efficient way to donate – saving time and cutting costs for the charity.