My best friend (20M) asked me (20M) out. I said yes. I didn't know it was a date.

I'm an idiot. I'll try to keep this short. Also, I was a bit drunk when this happened.
Noah and I have been best friends since 2nd grade. We live together with one other friend near the college we go to together. He came out to me when we were in high school. It didn't change anything. When we got to college, I came out as bisexual.
Last night we were hanging out with some friends and I had a bit to drink. When we were walking home I could tell he was nervous so I asked what was up. He asked if I wanted to go out with him. He literally said "do you want to go out with me". I don't know why, but I thought he meant as friends. Thinking back he made it really obvious it was a date, but I'm an idiot and I was still a bit drunk. I said yes. He suggested Saturday night. I said yes.
I left to go to the gym before he got up so I didn't see him. While I was at the gym, the friend we live with texted me that Noah was really happy this morning and she didn't know we were going to go on a date. She congratulated me and said we make a cute couple. This happened literally six minutes ago. I'm panicking right now. I don't know what to do. I love him but he's like my brother. I can't go out with him. How can I tell him I'm not interested without embarrassing him more than I have to? I don't want to mess up our relationship. We're supposed to have lunch together in a few hours. What do I do?
tldr: I'm an idiot. My best friend asked me out. I said yes. I didn't know it was a date. How do I fix this as easily as possible for him?
UPDATE: Thank you guys so much for the advice. I only saw the first 30 something comments until just now, and I'm blown away by how many people have commented to give me advice. I tried to take a lot of the advice I got, but some of it was contradictory so I couldn't do all of it. Thanks again to everyone who gave advice, even if I didn't see it until after we talked.
I texted Noah and asked if we could skip lunch and go home instead so we could talk. I tried not to make the conversation dramatic. I apologized to him for misunderstanding him, and told him I knew it must have taken a lot of courage for him to ask me out. I told him that he was my best friend, I loved him, and that would never change, but I had never thought about a romantic relationship with him. I apologized for ****ing up and getting his hopes up. He asked a few questions, like did I really not know he meant it as a date. He apologized for "****ing things up" between us, but I told him that I wasn't uncomfortable with him feeling that way about me. I told him that I understood if he wanted space to process it, and I would give it to him if he wanted, but I also told him how important he is to me and how much I love him. He said he didn't think he wanted space. He just wanted to try to move on with the secret out.
We've always been totally open with each other (well, except for him liking me I guess) and he said he was happy he wasn't keeping his feelings a secret anymore. We had a cheesy moment where I told him he could always talk to me, about anything. It was like straight out of a romcom. We talked for a long time after that. Apparently he's had a crush on me since before he came out in high school, and after he had a few drinks last night he decided **** it, why not try. By the time he had to leave for his class this afternoon, things felt normal. I don't feel weird knowing about his crush, and he doesn't seem to feel hurt or anything because of my **** up. Maybe just a little sad. I'll definitely keep an eye on how we interact and how he acts for a while. A few people warned me against accidentally flirting with him, and I'll try to keep anything like that in check.
I guess this is a happy ending. I know there are still things to be aware of, but right now it looks like the conversation went pretty well. Thanks everyone for commenting and supporting. It helped me not just with advice but also with calming me down when I was really panicky this morning.


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