I love martial arts. I taught kempo full time for a little over a year, but now i just do it for fun. There are a lot of arts out there so try a couple and see what you enjoy. A big part of what you choose will probably be based on the instructors you meet. Remember, if you're going to be committing serious time into your training, you'll want to do it with someone you respect and who's teaching style works for you. One thing i would recommend is not to be afraid of a little contact. I'm not saying find a place where you get beat up every week, but some people are extremely afraid of getting hit at all and its good to take one once and a while so you know what it feels like and how your body responds.
Personally i'm a big fan of American Kempo. Not only does it allow dirty fighting, it encourages it! Plus its an evolving art, so it was kind of an MMA (mixed martial art) before anyone had made up that term. Hey, even Chuck Liddell has a Kempo tattoo
(too bad it didn't save him a few weeks ago).
Overall the martial arts have taught me a lot more than just self defense. They've taught me a lot about myself, about what i thought my limits are and what i'm actually capable of.
If you want to get tuff fast, find a Muay Thai school, but be prepared to work your butt off, lots of respect to those guys.
If you want something that's just straight up self defense look for something like Krav Maga or anything similar. Arts that are entirely combat oriented.
Aikido generally(not all instructors are the same) places a lot of emphasis on the spiritual side of things, so if you're into that it can be interesting. If you're not so into it, it may be something you want to avoid.
Then again there's always ninjitsu