New shoes; big dilemma

Sorry for the long post, but I need to explain the question fully.  

I've been running with Asics 2130s for the past year.  I did one marathon, and I'm training for my 2nd marathon.  I lost a lot of toenails, but other than that the Asics seemed to fit "okay". 

Now that I'm running much faster, I thought I'd go in for a new fitting and see if I could get some shoes which fit better.  I left the store with Brooks Adrenaline.  

Now, with new shoes for comparison I'm 90% sure that the Asics were a bit too narrow for me.  The net effect was that my feet were twisted slightly outwards, limiting ankle flexibility.  Also with the foot twisted slightly up, the toes bent down, leading to toe blistering and toenail loss.

The Brooks fit seems to be a _much_ better, but there is a downside.  With my foot flatter and my ankle freer, the muscle effort is distributed differently.  I've got a lot of miles on my old, wrong, fit and I'm used to it.  Even on a 4.5 mile run yesterday, muscles and tendons I hadn't previously been using much (particularly around the achilles) were beat up and tired.  

It almost feels like I'm going to have to build up a lot of base miles from scratch.  But, I've got a marathon in less than 2 months.  

I'm terrified of trying this week's 16.5 mile run with the new shoes, for fear that I'll strain an underused muscle.  But I can't wait longer, as running an 18 mile run in new shoes would be even worse.  My plan for tomorrow is to run 5.5 miles in the old shoes, and then switch to the new shoes for the last 11 miles.

Based on the success or failure of that, I'll decide if I want to keep building up muscles for the new shoe fit, or if I want to dump the better fit for now and buy another pair of Asics to get me through the marathon.

Any advice?  Does this seem reasonable?