Oh, and just one more point. Your predicted time might well come down over the next few months so don't waste time worrying about what pace you want to go at on race day now. We all have times we'd like to be able to do, but you have to train at what you can do now and you can revise it later.
Yes, I'd say slow down your long runs. My 5k time is similar to yours, so these paces may be helpful to you: for my recent marathon my marathon pace was about 10 min/mile and I did my long runs between 10.30 and 11 min/mile.
As cougie says, if you do all your long runs at MP, you'll be wrecked by the time you get to the marathon. Also, for maximum training benefit, you want to do your long runs well below the lactate threshold. I know it seems counterintuitive to run slower than you intend to for the marathon, but trust the training plan - it works.