Like Gayle said, it really depends on what kind of mileage you've been doing and can comfortably handle. It also depends on your goals. If you're not trying for a sub 2:45 or so marathon and/or don't have a very good base, I think going over 24 miles in a training run is a bit much.
With that said, I think somewhere in between is a good approach. Given the 18 week constraint and assuming you're doing the marathon at the end of the 18th week, I'd do the following weekly long runs: 13, 14, 15, 16, 15, 16, 17, 16, 18, 13, 20, 14, 22, 15, 24, 18, 13, Marathon Race.
This will allow you to build gradually. I believe anything over 18 really taxes your body and you need a good two week rest afterwards. Again, this is relative to your fitness level and past training experience (an ultramarathoner might find 18 easy, but of course an ultramarathoner's probably been working up their mileage for years).
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