Let's say you're an 18 year old male who runs every other day, logging 12 miles a week.
Now fast-forward to your late 40's. Your eating habits are the same. To maintain your high school weight, how many miles a week would you need to run? Believe it or not, you'd have to multiply your weekly mileage 4 and 1/2 times, increasing it from 12 to 54 miles a week!
Unless you take up weekly double marathons in middle-age, it's impossible simply to "outrun" middle-age spread. That's because metabolism decreases with age. At 50 you just don't burn as many calories as you did at 18. Luckily, there are other factors in this equation. Gradually eating less is one way to make up the difference, since metabolism slows by about 100 calories every ten years. That's equivalent to a tablespoon of butter. To keep pace with metabolic changes, your daily menu at 40 could include everything you ate at 30, minus that tablespoon of butter!
However, cutting more calories every ten years gets trickier, and limiting food intake alone isn't the healthiest approach, anyway. Between our 20th and 70th birthdays, most of us lose about 30% of our muscle cells. Loss of muscle mass is the main reason metabolism slows down in the first place, since resting metabolic rate is directly related to lean body mass that's mostly muscle. Maintaining or increasing muscle mass through strength training ups your metabolism and burns more calories, even while you're sitting on the couch! Two or three sessions a week, using free weights or equipment at a gym, will do the trick.