I can relate to your situation, as I’m also 39 years old and trying to achieve at least one more lifetime-best performance before it’s too late. So the advice I’m about to give you is based on what I’m doing in my own training in hopes of peaking in middle age.
To begin, yes, it is reasonable, given your circumstances, to believe that you can still improve your half-marathon PR. If you were a 39-year-old hardcore veteran who had already run 16 marathons and logged more 100-mile weeks than he could remember, I’d say you had no chance. But you haven’t come close to doing all you could do in training to stimulate improvement, and by exploiting some of that untapped potential you can more than make up for the effects of age on your performance capacity.
It is clear that you understand there is no way to take your performance to the next level without training harder in some way. Increasing your running mileage is one way to do that, but not the only way, nor necessarily the best way. You could also keep your running mileage about the same and increase your cross-training, in order to boost your endurance and cardiovascular fitness without increasing your injury risk. You could also keep your running mileage about the same and make your key running workouts harder.
If you are fairly injury resistant, increasing your running mileage is probably worth a try. But don’t expect to perform much better on, say, 60 miles a week of running plus 60 miles a week of cycling than you have on 45 miles a week of running plus 100 miles a week of cycling. That strikes me as a wash. To stimulate another breakthrough, you’ll likely need to increase your running without reducing your cycling much, or else combine a shift from cycling to running with other measures.
On that point, I don’t want to assume that you are not accustomed to doing really hard workouts, but in my experience very few runners at our “recreationally competitive” level do really hard workouts on a regular basis. I’m talking about going to the track and blasting 12 quarters at mile race pace, running hill repeats until you’re literally lightheaded, and planning threshold runs that are so tough you actually get butterflies in your stomach before you do them. If you are doing anything less, then this is the single most effective training change you can make toward improving your performance. Increasing mileage is often the lazy, copout way to seek improvement. Making brutally hard workouts a consistent part of one’s routine is far more potent, although tough for many runners to face.
I’d also like to draw your attention to what is lost at our age: power, strength, top-end speed, flexibility, and mobility. Endurance, aerobic power and economy start to slide later. Therefore, the best way for a 39-year-old runner to slow the hands of time is to make a big commitment to strength training, plyometrics, sprinting, stretching, and mobility work. This is a major initiative of my latest comeback, and I strongly encourage you to follow my cue.
At age 56, triathlon legend Dave Scott is still fit enough to complete an Ironman in less than nine hours. For reference, the Hawaii Ironman course record for his age group is around 9:45. When I asked Dave how he had managed to hang on to his performance to such an unprecedented degree through the years, he cited his lifelong weightlifting habit as the most important factor. More than anything else you can do as a runner, strength training keeps the muscles young.
I hope these ideas are helpful. As a fellow child of the early ‘70s fleeing Father Time, I’m rooting for you!