Hi Homlyme, welcome to the plan. As a T2 you will know to be careful around reduced calorie intake and exercise and watch out for hypos. This diet can have a very quick effect. I went from a BG of 190 (around 10.0) to 120 (around 6.0) in the first 2 weeks. It scared me to death but soon got used to it. You dont mention meds. If you are taking any they might need adjusting over the coming weeks and you should speak to your diabetic specialist.
On the subject of exercise, and well done you on going for a marathon, I organise my exercise close to having eaten something, so about an hour after breakfast or lunch. If it has got late i.e. swimming at 4pm, I check my BG beforehand and possibly have a snack first. Always carry something with you – a glucose supply, some nuts or a piece of fruit or fruit drink just in case your BG drops.
Good luck.