I have to completely disagree with Jeff on most of his assessment. Sorry, Jeff. No disrespect intended. I have just been sawing lumber on my own mill for 8 or 9 years and have sawn my share of walnut.
There is approximately 1,300 BF of usable lumber in that log based on the international scale and the measurements you gave (
http://www.woodweb.com/cgi-bin/calculators/calc.pl?calculator=log_volume). If you saw it with a bandmill, your yield will be even higher. Not sure what it costs to have it milled in your area but I own a Timberking 1220 and charge $.25/bf. That would put your finished green lumber at about $.71 BF, not including any kiln drying. You can very easily air dry Walnut and I actually prefer air dried over kiln dried.
I agree with the others that it would be much better to have the log milled now rather than later but it is not mandatory to get descent lumber. Make sure you use a good quality endsealer (anchorseal, not any kind of paint) and keep the log out of the direct sun. Since you are heading into winter and you are in NY, you should be ok. If you were here in Texas, you might end up with some minor loss due to end checking.
I just finished milling 2,000 BF of black walnut that has been down and not endsealed for nearly 2 years. The sapwood was basically gone but no one wants that anyway! The ends were checked about 1.5 feet into the log on each end. Other than that, that was the only issue and the logs I had did not look NEAR as good as yours. A few years ago, I milled a walnut tree that had been down for 15 years and it was still like a green log in the middle!
One other thing, 36" is the upper end of any bandmill. If it is over 36" at all, be prepared to pay the sawyer extra to prep it to fit his mill.
This was all meant to give a different perspective based on my hands on experience and no disrespect is intended to anyone above who posted a differing opinion!