Keeping a diary of your food and exercise is good for several reasons:

• Trends - Are your times getting better? Are you running longer distances? Do you see a progression in your workouts? Did you have a bad run every time you ate clams the night before? You get the picture.
• Planning - By noticing the trends you will be able to plan ahead and increase the productivity in your workouts. Maybe next time you'll skip the clams the night before a big run.
• Motivation - Every once in a while I'll look back on a 2-3 month period, add everything up and say "I ran HOW many miles?" It can really get you psyched up to get out there for some more.

Here's a sample of some things you might keep track of:

• Date/Time - Time of day is very important in that it will tell you how your body reacts to different times of the day.
• Weather - It's good to keep track of this so you can see how it affects your attitude.
• Attitude - How you felt that day - both in terms of attitude and how the workout went. Were you tired, hungry, dizzy, brilliant, etc?
• Shoes - I go back and forth between 2-3 different pairs of shoes and never try to use the same pair two days in a row.
• Activity - Swim, bike, run, walk, weights, golf, frisbee, and anything else you want to track.
• Distance/Time - Both how far you went and how long it took you. This will also let you guage pace.
• HR - Most HRMs have an average heart rate function. This is very useful to track how hard you are working versus how good you feel.
• Meals/Snacks - Not only what you eat, but what time you eat it.