I have been running for 6 years now, consistently. I run an average of 20-30 miles per week, but have sometimes peaked at 50+. I have been running in minimalist shoes from Nike free, to Merells for the last 3 years. I have never really done any runs more than 13 miles in them however. I recently bought a fancy pair of Asics thinking that it would be a nice change for the increase in distances I will be implimenting, but they left me running back to my Merriels very quickly. I have done a 10 mile race in these Merrells on roads and really didn't experience any issues. I just feel so comfortable in these shoes, but I just don't want that 26 miles on the pavement to be too detrimental. Please any advice or personal experience would be great! As of right now my plan is to run the marathon in Merrells.
Hard to tell, just from the info you gave. I have run in Kinvaras, have done a couple half-marathons in them and I really like them. But based on the half-marathons, I know they wouldn't work for a full. You really can only go by your experience. How soon is the marathon? Can you get in a long run (16+) in the Merrells? I personally think it's around 16 - 18 that you really start to feel any shortcomings in the shoes. You don't say which Asics you bought and it doesn't sound like you gave them much of a test.
Also interesting that you sometimes hit 50+ (with no long run) while typically running 20 - 30. That's a big difference. But that's another matter.