When he does, his mind will be taken back to the moment 18 months ago when he nearly slipped into a coma because his blood glucose levels were dangerously low.
Blurry eyed, tired and with an unquenchable thirst, Forrest had wondered what was wrong with him.
Just a year earlier he had competed for Australia in the marathon at the World Championships. Though he knew something wasn’t quite right, he didn’t think for one second it was diabetes. He was, after all, an elite and very fit distance runner.
“Looking back now the symptoms were there,” said Forrest. “I just didn’t know that a professional athlete could get it. But my vision was getting very, very bad, I was drinking a lot and having to go to the toilet a lot. I was getting really tired, too. But as an athlete, if you get to the end of the week and don’t feel tired, you feel like you are doing something wrong.
“I kept telling myself I would be OK but when I eventually did go to the doctor, they found my blood glucose levels were so low that I was that close to slipping into a coma It was basically running that was keeping me alive. I was running 100 miles a week and then because of niggling injuries, I dropped down to 30 miles and that’s when it hit me bad.”
Forrest’s life changed the moment he was diagnosed. From being an athlete solely focused on running faster, winning medals and representing his country, he now has a different motivation.
“My life changed instantly,” he said. “As soon as I read up about the disease, I looked at it in a positive way and realised that as well as running fast and representing Australia, I could now aim to help others with type-one diabetes.
“A lot of people don’t understand the disease and think that you can’t continue training or continue being the person you were before, but I want to show people it is manageable and running actually helps.
“I feel like I’m not just running for me anymore, I want to be an inspiration and to get other type-one diabetes sufferers out there and running.
“Running is a pure sport. All you need is a pair of shoes and that’s it. You don’t need a lot of money and it’s a great way to manage diabetes.
“I truly believe that I was chosen by this disease. It is my motivation to show people that you can manage it and for children with type-one to look up to me.”
Forrest, who is coached by former London Marathon winner Steve Jones in Boulder, Colorado, has not given up on his other dreams too, of running faster and representing his country again.
His personal best marathon time of 2:14:36 was set in Houston in 2012 and he believes he can attack that figure in London this weekend as he tries to win a place at this summer’s Olympic Games in Rio.
“Training has gone well,” said Forrest, who will be running his first marathon since being diagnosed a diabetic. “There have been a few ups and downs but with diabetes that is to be expected.
“There’s nothing like racing and I just can’t wait to go out there and run. I have always wanted to run the London Marathon and why couldn’t I make that Olympic team? I believe anything is possible.”
Forrest was inspired to become an elite athlete when, aged 16, he was chosen to be a kit carrier at the 2000 Sydney Olympics and carried Haile Gebrselassie's basket before the Ethiopian legend's epic 10,000m duel with Kenyan rival Paul Tergat.