As you naturally age you will burn less calories. So as you continue to eat the same way and the same amount you will have to up your exercise to impossible limits. What needs to change is your diet. No sugars and lots of healthy fats.
Here is an excerpt from the book I mentioned.
Why We Get Fat: And What to Do About It by Gary Taubes
http://amzn.com/0307474259
>The authors were Paul Williams, a statistics expert at the Lawrence Berkeley National Laboratory in Berkeley, California, and Peter Wood, a Stanford University researcher who has been studying the effect of exercise on health since the 1970s. Williams and Wood collected detailed information on almost thirteen thousand habitual runners (all subscribers to Runner’s World magazine) and then compared the weekly mileage of these runners with how much they weighed from year to year. Those who ran the most tended to weigh the least, but all these runners tended to get fatter with each passing year, even those who ran more than forty miles a week—eight miles a day, say, five days a week.
This observation led Williams and Wood, both believers in the doctrine of calories-in/calories-out, to suggest that even the most dedicated runners had to increase their distance by a few miles a week, year after year—expend even more energy as they got older—if they wanted to remain lean. If men added two miles to their weekly distance every year, and women three, according to Williams and Wood, then they might manage to remain lean, because this might mean expending in running the calories that they seemed fated otherwise to accumulate as fat.
Let’s see where that logic takes us. Imagine a man in his twenties who runs twenty miles a week—say, four miles a day, five days a week. According to Williams and Wood (and the logic and mathematics of calories-in/calories-out), he will have to double that in his thirties (eight miles a day, five days a week) and triple it in his forties (twelve miles a day, five days a week) to keep fat from accumulating. A woman in her twenties who runs three miles a day, five times a week—an impressive but not excessive amount—would have to up her daily distance to fifteen miles in her forties to retain her youthful figure. If she does eight-minute miles, a nice pace for such a distance, she’d better be prepared to spend two hours on each of her running days to keep her weight in check.