What Phil said!
But with the added comment that, if it occurred to you to try, then it is definitely worth the effort. Even if this one doesn't work out, you will learn from it and the next one will.
So, if you cut it into a (very) oversized blank, can get a serviceable stock out of the heartwood (dark center), and see no rot, then let it air dry for a year indoors and then get to it!
Note: you might want to wax or prime or otherwise seal the end grain right away. The open ends of the pores will be where the greatest amount of moisture exchange takes place, so that's where many splits and cracks originate. They may appear elsewhere as well, in which case it's an unsuitable piece, but the ends are most prone to 'checking.' You can minimize that by sealing the ends.