First off, if you're looking for self-defense, do NOT look towards Martial Arts. Very few will teach you the mental skills needed to survive a street confrontation. You can have the best moves in the world, but they will do you no good as you shit a brick when someone has a knife or a gun at your nose.
I studied under Tony Blauer for 3 years. The stuff he teaches is invaluable, and you can find it online.
For the physical aspect, assuming you've let the situation escalate to that, look towards Systema or Krav Maga, make sure they teach conditioning (teaching how to take a hit). Muay Thai is awesome, as are BJJ and Kempo, but you may find them too physically demanding at this stage in your life.
Disclosure: I have been a 9 year Muay Thai practitioner, with 30 years of MA and self-defense training.