I'm stuck with a spare tire (donut), what should I do?

On my way to work this afternoon I got a flat and I only had a donut to drive on. I just got 4 brand new tires in November and they have an 80,000 mile warranty on them but I'm petrified they won't take my tire back and even if they do, it's going to be a while because I got them on the internet. There's a slit on the inside wall of the tire, I didn't hit anything, it just went flat and I still don't know how. I have to get to my bf's house this weekend so he can look at the tire since he bought them and the only way to get there is the highway that has a speed limit of 65! The man who helped me when I got the flat said not to go over 50 with the tire, and so did that cop that stopped to make sure I was alright. My bf told me he drove 80mph on the donut when he had that car and had to use it to get home from school once. I don't plan on going 80 when going to his house, but can I at least go the speed limit of 65? I know nothing about cars or tires since this is my first car and the first time I ever got a flat. Any help would be greatly appreciated.