My ex lost 80 pounds and went from unable to run to running ultras and she found that she had to alternate between walking and running in the beginning to keep her heart rate in the lower thresholds. On the converse side, maybe integrating some strength training or speed workouts or stair climbs will help you develop a strength reservoir to make it easier to run at a low heart rate.

Now that you've recognized how gentle a low heartrate feels I'm sure a conversational pace is totally adequate for staying under your lactic threshold (but if you could sing you're going too slow!)