Day 1.... barely even started

So today was the first day and it started out great, I had a special k breakfast (nom nom), a salad for lunch and fruit and veggies throughout the day. Even my supper was a yummy stir fry filled with veggies and even went for a walk, but then the evening hit... my downfall every time. Chris made some banana muffins so i ate half of one, not too bad i could handle that, but  then i went out to visit Sis. Cerny and Steph. Sis. Cerny made me a cake :o( and of course i had to have some (i didn't want to be rude). I also didn't want to go empty handed so i had brought some carrot muffins and had some of that too. So sadly i can't call today a success but definitely a stepping stone in the right direction, if only i could skip the evenings. So my question i have from today is how do you say no when you're at someones house? Is there a polite way? Or do you just eat it and continue on?