The hardest I've ever been hit was by a 57 year-old Japanese martial artist who nearly broke my leg with an arm block, a truly amazing feat. If you truly understand the techniques and bio-mechanics, age matters not in the martial arts.
This interesting book covers attributes of fitness, flexibility, power, reflexes, coordination, speed, and endurance with a focus on older martial artists. There is pretty good coverage on the effects of aging and how to train hard without hurting yourself. The sections on injury prevention, self-care, and over training are worth the price of admission. If you've never practiced martial arts before, the sections on progression in the martial arts, sparring, and forms are also very good. They are not particularly useful if you've been doing this a while, however. The introduction and the first few chapters are very basic indeed. The section on weapons forms near the end is a nice bonus though.
All in all the vast majority of material in this tome has been covered better in other books, some written by Kim himself. The target audience is the middle-aged beginner who is interested in pursuing an education in martial arts. If you are over 40, new to martial arts, and only going to purchase one book, this is it.
If you have been training for a while I wouldn't put this too high on the priority list. I'd recommend "Ultimate Flexibility: A Complete Guide to Stretching for Martial Arts" by Sang H. Kim along with "The Fighter's Body: An Owner's Manual: Your Guide to Diet, Nutrition, Exercise and Excellence in the Martial Arts" by Loren W. Christensen instead. Yeah, it's two books rather than one, but I think that combined they have the depth advanced practitioners require.
Author of Surviving Armed Assaults, The Way of Kata, and Martial Arts Instruction