Hi James, and welcome,

Ninjitsu, like any other martial art, although not a common base for striking (kick boxing, muay thai) or ground game (BJJ, wrestling) but due to the nature of the sport (Mixed Martial Arts) anything really can be used to improve your over all gain.
I dont really have any experiance in Ninjitsu so im not sure what a training session would be but for the marines and cardio i would recommend boxing really, which also gives you a great base for moving into the cage.

Hope this helps