Run Less, Train More
Runners age differently from other endurance athletes. The best swimmers and cyclists in their 40s and 50s are the same athletes who were the best in the world in their 20s and 30s. But the best masters runners are typically late starters, while former professional runners who continue to run competitively into their 40s and 50s tend to slow down a lot. One explanation for this phenomenon is that the repetitive impact of running reduces the elasticity of the soft tissues in the legs over time, and takes a lot of bounce out of the stride. This is something that athletes in non-impact sports such as swimming and cycling don't have to worry about.
More: Why Runners Need Spinning?
A tried-and-true way to preserve your legs after you turn 40 is to replace some of your runs with workouts in a non-impact aerobic activity, such as cycling. The more similar your running alternative is to running itself, the more crossover fitness benefit you'll get from it. Recently many masters runners have turned to outdoor elliptical trainers such as the ElliptiGO bike, which provides a very running-specific workout without impact. Last year Simon Gutierrez finished second in the masters division of the USATF Cross Country Championships off a training program that combined running with ElliptiGO rides.
In order to continue to perform well in running races, you need to keep running at least three times a week. But you may not need to run any more than that. The rest of your workouts can be in non-impact aerobic activities. Because these other activities don't impose as much wear and tear on the body, you may find that you are able to train more comfortably with a hybrid training approach than you could when you only ran.
Defend Your Speed (But Not Too Hard)
Running fitness has many components, and they don't all diminish at the same rate with age. Speed, for example, tends to slide earlier and faster than endurance, which is why elite runners tend to move up in race distances as their careers progress. It makes intuitive sense to counteract the effect of aging on speed by doing more speed work. However, along with speed itself, tolerance for speed training also diminishes with age, so you need to be careful. If you have made a proper commitment to speed training in your younger years, increasing that commitment is not an option to defend your speed against aging.