The best way to make sure that you enjoy your hike in the Grand Canyon is by being prepared, mentally and physically, for the challenges that this magnificent place delivers. I have seen people in the prime of life and in excellent physical condition have a horrible time because of a bad attitude and/or poor preparation. Most of the time it is because they have totally underestimated how difficult the canyon would be and overestimated their own abilities.
On the other hand, I've seen elderly people who looked like they've spent more than their share of time in front of the television who handled hiking the Canyon fabulously. All because they were better mentally prepared for the trip and followed certain guidelines for ensuring a good time. You're already a step ahead because you are doing your research now!
So do yourself a favor and train, both your body and mind. It's much more beneficial to underestimate your physical fitness and abilities than to underestimate the Canyon. And ALWAYS keep a positive attitude!