I'm a newbie myself, so I'd like to respond to just your first question, since I also had some experience with Tae Kwon Do. I'll leave the other questions to better qualified people.
Any martial art, any sport can be either physically demanding or not depending entirely upon your application and effort. What makes aikido different from other martial arts like Tae Kwon Do(and which probably contributed to your impression that it's less strenuous) is the manner in which it's demanding.
As an adult male, unless you've consistently done some kind of conditioning/flexibility training, you're not going to find arts like Tae Kwon Do easy to do. You'll find tendons and muscles and joints in weird places screaming in protest at sudden exertion. It requires movement beyond what 'normal' life calls for. In Aikido, on the other hand, movements are 'natural' and rarely go beyond what any normal person can do. But then, so does running. Think you can run the marathon straight off? Probably not. But you can still walk to the local convenience store and back, right? That's pretty much what Aikido will be like. You can start off with just the ability to walk and the ability to stand up. How much you improve will depend entirely on your own commitment.
Good luck and hope this helps.
Kyung Yul Lee