You could also use some of that extra energy you freed up by slowing down to increase your overall weekly running mileage. Gently increasing the amount of easy running you do is the simplest and surest way to improve your fitness. There is a reason elite runners run more than 100 miles per week. You might not have the time or the durability to run quite that much, but you certainly could run more than you do. And if you think you couldn't—well, slow down and think again.
The only reason elite runners are able to run more than 100 miles per week is that they do most of their running at relatively low intensities. In fact, elite runners run slower relative to their abilities in designated easy runs than non-elites run relative to our abilities. I've enjoyed opportunities to run with a number of elites, including Kara Goucher and Haile Gebrselassie. It's never been any trouble for me to keep up with these folks, despite my lesser abilities, because they generally don't put that edge into their easy runs that we put into ours.
Imagine how comfortable it must be for someone like Gebrselassie—who has run a marathon at 4:43 per mile—to coast along at 7:15 per mile. A certain kind of discipline is required to hold oneself back like that. But the rewards are great. Running is more enjoyable, you are able to perform better in key workouts, and you are able to run more and improve in that way if you so choose. Give it a try!race.