*raises hand* Former kendoka here. Practiced it in my teens, but stopped when I was around 18 when I moved away and couldn't find a good dojo nearby.
I've never really practiced any other martial arts, so I can't offer any personal comparisons, but generally speaking kendo was quite effective at keeping me fit. I'd imagine it may be a bit more strenuous than other martial arts, because you're swinging around a sword rather than just your arms and legs, but again, I don't know from personal experience. If you're worried about it being too tiring, you could always gradually work your way up. The dojo I went to would have a special section for beginners, who would be dressed in their own exercise clothes, and they'd receive instruction from one of the more senior students. After a few months in the beginner's section, the sensei would give his permission for you to graduate to the general section, which would require you to get your own hakama (loose robe-like trousers), gi (the top), and bogu (the armor).
I practiced it for about four years in total, and make it to 1st kyu, which is the rank just below shodan. Participated in a handful of tournaments, but always lost in the first round, because there were so many people so much better than me. But that was ok, because I was just a beginner, really, so I didn't mind losing.
All in all, if you're interested in giving it a try, I'd say go for it. I do miss my kendo days sometimes, because that was probably the fittest period of my life, and there really is something therapeutic about hitting people with, and getting hit by, a big bamboo stick.