Before shedding her weight she would typically start the day with four slices of peanut butter on toast, a sandwich, crisps, chocolate bar and a can of pop fizzy drink for lunch and a pizza for tea.
And throughout the day she would munch away on chocolate, crisps and jam tarts.
STAYING HOME TO SECRET EAT
"It was a cycle - the bigger I got, the less confident I was about going out with my friends so I'd stay in and secret eat instead," Ms Marshall said.
"At school I'd sit on the field in protest in PE lessons and refuse to join in.
"What happened on the tube back in December 2015 - on two separate occasions - was humiliating.
"Having to explain that I wasn't actually pregnant was bad enough, but I also knew I probably wouldn't fit in the seat they'd offered me anyway.
"My confidence hit rock bottom at that point - I'd go to bed early, get up late and I didn't really leave the house much at all.
"I hated looking at myself in photos and I was sick of never being able to get any nice clothes."
After seeing how distraught she was, Ms Marshall's grandmother Pam took her to a slimmer's group in north London.
"I DIDN'T KNOW HOW TO BOIL AN EGG"
The Middlesex University student began following Slimming World's Food Optimising eating plan and says initially her nan, who she lived with at the time, did the bulk of the cooking.
"I didn't know how to boil an egg when I started losing weight, so I'd been pretty dependent on takeaways - I usually had at least three a week and I was known to get through a 12-pack of jam tarts regularly too," Ms Marshall said.
She started her weight loss journey hiding in her front room to complete workout DVDs, including Jillian Michaels' 30 Day Shred. The US fitness mogul is known for her hardcore technique and has been branded "America's toughest trainer".
But as her confidence grew, Ms Marshall plucked up courage to don a swimming suit and hit the local pool. Now she trains at the gym between three and five times a week and said her dieting had 'completely changed' her lifestyle and given her a huge confidence boost.
"To begin with, I was really shy and I didn't want to go out in gym clothes, so I would close the curtains and do workout DVDs in my front room," Ms Marshall said.
"Once I'd lost about five stone [31kg], I felt confident enough to go out, and it was a pretty big deal for me to put a swimming costume on.
"Swimming was a childhood love of mine which I hadn't done for years, and I would wear men's trunks and a T-shirt to swim on holiday, so that was great."
With her partner of seven years Mark, who she now lives with, she went on a hiking and cycling holiday in Florida recently, where she wore shorts for the first time in her adult life.
Now weighing 76kg and a size 12, Ms Marshall said her new-found confidence had let her enjoy socialising again. She even signed up for a sky dive to celebrate achieving her target weight.
"Before, whenever I walked into a room I would feel as though everyone was looking at me and thinking: 'Wow, she's a big girl'," she said.
"Now I can walk into any room standing up straight and with my head held high.
"I always thought I was destined to be overweight forever, but I've changed my destiny and if I can do that I believe anyone can.
"And I definitely don't have to worry about anyone offering me a seat on the tube because they think I'm pregnant anymore - while that usually means I have to stand, it's a pretty good feeling."
BEFORE MENU
Breakfast: Four slices of white toast spread with butter and peanut butter;
Lunch: Pre-packaged sandwich, crisps, chocolate bar and a sugary fizzy drink;
Dinner: Takeaway, such as pizza or chicken burger;
Snacks: Sweets, chocolates, crisps, jam tarts.
AFTER MENU
Breakfast: Overnight oats - porridge oats layered with lots of frozen berries and quark;
Lunch: Couscous or pasta salad, or 'pizza' omelette with salad;
Dinner: BBQ pulled pork with cabbage and green beans, or homemade kebab with a wholemeal pitta bread, Slimming World-style chips (baked in the oven in low calorie cooking spray) and salad;
Snacks: fruit, high-fibre cereal bars, popcorn, small bar of chocolate or Twister ice lolly.
