Of course you should use a full pack to make sure you have the right fit for comfort, and certain items like shoes should be broken in, but I believe it's more important that you improve cardiorespiratory & muscular endurance to keep from getting sore & worn out on a hike than it is to practice by toating a full pack around.

I'm not an expert or as experienced of a backpacker as many others that have responded here.
I've spent the last nine months conditioning myself to be trail ready. I've only had time for day packing and I haven't traveled any more than 10 miles on foot in a day, but I typically do a 7 - 9 mile day trip twice a month now; however, I don't have to take much in my bag for these short day hikes.

I'm 5'11" & have maintained 140# throughout my training. My goal is to gain muscle mass and more importantly improve my cardiorespiratory and muscular endurance.
I started on my cardio by walking 1.5 miles 3 times a week, then I stepped up to jogging the same distance, and now I'm jogging 2.5 miles & running full speed for another 1/3rd mile 3 times a week. I do change it up by running down local nature trails for roughly the same distance, but he changes in elevation on these trails increases the difficulty & I cannot not always jog all the way.
For my muscular endurance I do simple things on a mat at home like sit-ups and push-ups, but I also do pull ups & chin ups.

These runs take me about 45mins, and I believe they have paid off. The day after my 7 to 9 mile day-hikes I am not sore, and usually I feel full of energy the day after a hike.
It's also worth mentioning that I hunt with hounds and I typically walk through the woods another couple of miles 5 or 6 nights a week when I'm coon hunting, but this is much more relaxed.

Anyway that's what I have to share from my own personal experience.