Since your plan has to have a purpose, you need a goal.
So what's your goal in physical exercise?
I put all my races in on a calendar and started filling out the year.
The weeks before the race are filled with buildups and workouts culminating in a (hopefully) successful race.
The weeks after have a little lighter load to let my body recover.
I print out my calendar and it becomes my road map for the year. Although I don't do everything on the exact day that I have planned, I try to get every scheduled workout in. I run in the rain, in the snow, in the heat. I run on vacation, when I'm at girls camp and when I haven't left the house all day.
After each run I record what I did. The distance, the time and the average pace (thank you Garmin!) on my calendar.
Then I get out my training journal. I put the same information in there plus if I pushed the jogger (bringing down my average times which always bugs me a little), if I ran on a treadmill, what shoes I wore so I can rotate them, and if I ran with friends. I also put any aches and pains I am feeling and how I generally felt during the run.
It may sound fanatic, but my excessive number counting and journaling does many things.
It keeps me on target and successful in my goals. Since I have times of rest built in I am able to avoid injury. Also as I track what my aches and pains are I can see patterns and tell if I need to change out my gear or start stretching a different way.
It's also fun to add up how far I've run and see my times improve from month to month.
Planning is an important part of exercise. Though you may not want to be as detailed as I tend to be you will find more success as you document your exercise routine.
