I do xfit. been at it for over a year. Im 42, and definitely not the same as i was in my 20's, but I have changed 180 degrees. Our coaches are very into safety at our gym. They will actually not let you do too much, everyone has a progression and all workouts are scaled to the individuals abilities. Yes, time is important, but they will slow you down or make you take weights off if you are not doing correct form.
Not all boxes are created equal and not everyone gets the same exposures, but Id say I'm very happy with my xfit experience and will keep doing it until I physically can't. Also, our coaches preach healthy eating habits....whatever one you like, just make sure it fuels your body not just fills your stomach.
I believe my biggest gains in my backcountry hunting have come from changing my diet. Im doing more and feeling better because of my eating habits, or because of the things i have given up. I don't DIET, I have changed my diet. I actually eat like a horse now, 5 times a day. Its crazy how my metabolism has just woke up and gone nuts.