How can a Pure Titans recover the full body after extended period? It is said that Ymir spent about 60 years in Pure Titan form. It is also said that Pure Titan only have original subject's spine instead of full body like one of Nine (see ref [25] here or search word "spine" if number shifts).

So, how exactly she could recover back to complete human with brain, conciousness and memories while turning into one of Nine? Other transformations explicitly shown or mentioned so far generally only take less than hour between turning human into Pure and upgrading to Nine, so it less of a concern, but 60 years? Shouldn't pretty much of whatever was original Ymir be long lost by then?
There is no specification on the manga or anime about that. But it seems that any "pure" titan as long it gets the spinal fluid of 1/9 titans is able to go back to being a human no matter how long it takes then in their pure titan form. Yet again, all of Ymir's people seem to see the invisible roads or at least the people that turn into one of the  1/9 titans. Time pass different in that place, seems to take years or thousands of years[forever]. Ymir, just mentioned that it was like a never ending nightmare to be in the pure titan form.  

Another way of putting it is, all pure titans are female eggs, and the 1/9 are the sperm, when of of the 1/9 titan gets eaten by an egg, it becomes a human, no matter how long it was in that place.