Never ever think that meditation and any kind of energy work alone can be a substitute for sweat, cramps, bruises and overall hard graft in 'hard' Japanese martial arts like ninjutsu or karate. Ok, you can call yourself a 'spiritual warrior' and do all kinds of spectular mental exercises in your head in the comfort of your home and that's fine but they won't help you at all in a dojo. They just get in the way when you practice your basic stances, punches, blocks and kicks because they just slow you down. Once you have the rough outline of your techniques down pat after training several years, concentrating too hard would simply tense you up - that's for raw beginners. Zen is the only way to go here: maintaining maximum mental emptiness, not-thinking (because even thinking is too much of an effort if you're exhaused, after all), reacting instinctively without thinking, and the easiest way to achieve such a state of mind are (deliberately) mindless repetitions so beloved of traditional Japanese martial arts masters. Shuffling in excruciatingly low stances one dojo-length after the other until your legs scream for mercy, and you'd have to be a master of inner Chinese martial arts to visualize all kinds of channels or chi points to relieve your muscle fatigua. Even your blissfully empty Zen mind will go straight out of the window in free sparring when some screaming guy comes at you hammer and tongs as if trying to tear you a new one; in fact, that's the weakness of ultra-orthodox martial arts - no free sparring, only scripted exercises where two partners cooperate (and afterwards feel invincible, poor deluded bastards)(and aikido is for pussys btw, I unashamedly admit that I'm prejudiced there

).
The typical low ninjutsu back stance (even lower that Shotokan's
kokutsu-dachi) will give you thighs like treetrunks after two or three years but mere meditation won't. If you're serious about ninjutsu, you'll also practice climbing, and it will take years of consistent practice to become good at it (and btw, aren't you way too heavy for that right now?). In practical martial arts, brawn will win out over brain any time. A taiji master against a guy who is just one year into his muay thai training? No contest. If you've ever been on the receiving end of those devastating low kicks and had a hard time climbing stairs for weeks, you'd know what I'm talking about.
If you want to immerse yourself in Shingon Buddhism, practise your mudras or other mystical ninja methods, be my guest. I'm convinced such an approach is a valid spiritual path but don't ever think you're ready for a physical confrontation - the archetypical concept of being a warrior can be useful metaphors but whenever somebody threatens you with violence, run away as fast as you can. If you want to learn self-defence, take up Krav Maga, for example, but meditation & energy work alone can never prepare you for such potentially lethal real-life situation. Never ever.
And now for some light relief (althoug the depiction of dojo life is painfully realistic):