It’s amazing how well your comment resonates with my current thinking about exercise. For precisely the reasons you described, I’ve started adding more slow, easy work-outs into my weekly routine, while reducing the intensity of my main work-outs. For cardio, I still do three runs per week, but only one of those runs is for speed. On the other two I keep an easy pace, which I define as a level that allows me to breathe comfortably through my nose. But in addition to the runs, I’ve added regular neighborhood walks, low-impact body weight exercises, and even swimming (though I’ve only gone once so far). Then on my weightlifting days, after I’m finished lifting, I do 30 minutes on the exercise bike at a fairly easy pace. I haven’t been aiming for a particular heart rate, but the sensor on the bike tells me that I’m averaging around 140 beats a minute, which sounds about right according your method (I’m 34).
By reducing intensity in all but one run a week, I have a greater capacity for more low-intensity cardio, which I’m finding excellent for stress levels. The low-intensity work doesn’t provide the same kind of rush as the high-intensity does, but it doesn’t lead to a crash either, which is probably more conducive to long-term well being.
I’ve even extended this philosophy to the weight room. My routine over the last several months has been in the 5-8 rep range for each set, occasionally even dropping down to 3 reps when I want to increase weight. My strength gains were good, but the heavier weights were really starting to wear me out. I’ve decided to switch to reps in the 10-20 range (depending on the exercise) using lower weights. My goal is to reduce the stress-load of the work-out, while also developing more muscular endurance, which I’m lacking because of my previous focus on low reps.
Good luck on your new running program. It’ll be interesting to see how much you can increase your speed even while keeping your heart rate below 140. I’m sure building up your base in a controlled way will be more enjoyable than killing yourself every run, which is probably what burns out so many beginning runners.