this is your shoe for sure. it's narrow and the tread is the best i've ever used for traction.

This is a narrow cushioned trainer that would more than likely fit your need perfectly. I have run in this shoe quite a bit but I have not run 50K in them. They are definitely worth a try and they have a little more cushion than the Fireblade. Good luck!

If I had any say in it, I've done several 50ks in the La Sportiva Fireblades, and they are a narrower fit compared to other shoes I've run in: http://www.backcountry.com/store/LSP0075/La-Sportiva-Fireblade-Trail-Running-Shoe-Mens.html It has a "low-to-the-ground, fast" feel (feels a lot like a trainer), and grips like an animal. This probably isn't a heavily cushioned shoe, but I run with this shoe with superfeet insoles. I haven't run in this shoe in particular, but I've run in the Montrail Hardrocks and the Continental Divides and I like the La Sportivas much more because it feels "less clunky" and more race-ready I guess. For running on dirt and grass, I have to send my shout-outs to the fireblade. It's a beast! Out of curiosity, what 50K are you planning on running?