You'll be fine. 12 miles RT isn't really that much. Just do a steady pace. If you go to slow you'll never get there, and your butt will be in contact that much longer.

I put on 35 pounds and hadn't been on the bike since November. Just last week I started commuting again and I'm already in decent shape, if not the shape I was in 4 years ago when I'd do 50 mile rides without giving it a thought. As I already said, a steady pace will be better than a slow one, and you'll be OK in a few weeks. You don't say how old you are, or what your weight is. These will factor in. (I'm 56 and not giving in to being closer to 60 than 50.)