I like to limit my weekly touring miles to no more than three times what I normally ride at home. I have just found that anything over that leaves me feeling brutalized rather than strengthened. If you want to average 150 miles per week, you need to get more saddle time. Also, I try to not raise my weekly mileage by more than 5-10% per week. So, from where you are I would want 10 weeks to prepare.

If you are young, go for it anyway. If you are an old fart like me, try to find a way to take more days. I've gone from just commuting (50 mile round-trip) to averaging over 100 miles per day on tour, but my body always wanted a break after about 25 miles and my bum was usually no longer on friendly terms with the saddle after 4-5 days.