One could do 150 a day if they biked for 10 hours a day, but most touring people like to go at a bit more relaxed pace. 18 miles a day is way different than 100 or 150. That's 700 - 1050 a week compared to the 108 you're doing now. I ride about the same amount per week as you, and I don't think I could do 150 two days in a row. Maybe 100, but I don't think I'd enjoy it very much. Most people that I've talked to typically do around 50 miles a day, some a bit more. Touring is supposed to be fun, not a brutal training exercise. Those come in the months prior I think you should train a bit harder, try to do a couple 50 milers a week if you have the time.

And good luck