10/09/2013 08:07 BST | Updated 09/11/2013 05:12 GMT

From Chelsea Buns, to Iron Buns

I recently turned 36, and don't get me wrong - I adore my thirties, but tipping the scales closer to forty, I looked over my soft curves with an extra portion of arse, and thought - It's now or never.

My twenties were a ball of confusion, random sex, and insecurities, so I wouldn't want them back for love or money, but, I was a great deal slimmer. This was partly due to being depressed; I didn't have an appetite for much apart from sad face and Sylvia Plath, coupled with my high sex drive, in desperate bid to get off and let happy chemicals flood my brain for once, by thrusting off the pounds, yes! YES!

Then in my late twenties the relationship that shaped me and lead me back into writing, also saw me at my thinnest ever due to being shagged 24/7 and feeling totally miserable slash paranoid most of the time. My hipbones skulking across the floor with my pert bottom to follow, I stayed super slim Sadie until I sat down to write the book, only moving from the sofa to the kitchen for cake. Hello 10-kilo weight gain! I was so deliriously happy at being single again I just didn't care that much; and for me being slim and happy have actually never been linked, until now.

I have reached a fever pitch of happiness during the last 3 years. Feeling more fulfilled and balanced than ever, totally at ease with life: I want my body to follow, firmly. Post birthday I decided - it's sinking boobs or swim, and once I commit to something, like travellers in a car park, there is no moving me. Sure I've styled out my weight gain with red lips, good hair and cool hats to distract from my landside of a waistline, but I KNOW what's underneath, plus I want to be able to strut around in nothing but over the knee socks this winter (at home, obvs) and feel good.

So I'm six weeks into my sparkly new routine, and it's starting to show. I am now a dedicated runner, and gym goer, switching up the two throughout the week. Parts of my old body are slowly emerging; my stomach doesn't move independently when I run, I DO have some upper arm definition coming through, and my infamous thighs are SLIMMER and FIRMER! Already, I can no longer imagine life without exercise, like Vogue it has become a Sadie essential. I am on a new path, which gives me a natural high similar to the shallow sex in my twenties but with no one else involved and much more feeling. I mean, this is a major achievement for me, a gym bunny - and loving it! Next thing you know I'll have a boyfriend: great calorie burner. I have also totally overhauled my diet to Hollywood standards, and I have to say, it's love, another shocker. No longer reaching for carbs and treats, almonds are my passion with a warm lemon water to match, and I am not budging from my Doutzen Kroes worthy routine. Once I'm in, I'm in: a man, a cake, a kiwi, end of.

Although I am still a health novice with my body at that 'looks okay in Lycra, not fit, but not too flabby' stage of my physical metamorphoses, I sense there will be no lapse back to my pre LA lifestyle via SE1. I normally feel ready to take on the world as a chubby wonder, but to take on the world with a busting body? Unstoppable. No longer picking up one's stomach or feeling your whole arm wave back as opposed to just your hand, lifts the soul higher than indulging in a Crispy Crème or five ever did. And with 2014 set to be full of engagements, book promo and travels, I am just in time to get my body press ready and not dread the squashing of body parts into support pants, only to pop out somewhere else and ruin the illusion of an 'okay body' one aspires to; I'm already on track to F.I.T - Fabulous Is This! (points to ultra firm slick bod)