Obviously Ukemi is quite essential to the practice of Aikido, I mean that it's not something special just to Aikido, or something that can only be explained by your Sensei. Of course, once I start, there will likely be changes that have to be made, variations that has to be learned, and so forth, but I see no reason why I can't start the basic practice myself
Of course you can -- you might find it counterproductive, is all. Ukemi and kamae are foundational skills, and there are differences in how they're done in different aikido dojos, let alone between aikido and other martial arts (we had a fella come to train with us with a background in jujutsu, so he said, and not only was his ukemi different, for our purposes it was almost nonexistent). It can be very, very hard to change the way you do a basic skill once you've learned it one way, which is a good argument for waiting to learn it from your Sensei rather than trying to teach yourself. Rather than trying to teach yourself any aikido skill (only to learn on day one, most likely, that you have to change it), I'd work on the conditioning that will make acquisition of such skills easier. But that doesn't seem to be what you want, so suit yourself.