Training different energy systems requires different approaches.
For base aerobic training, your heart rate is generally fairly low - below 80% of your lactate threshold. That feels pretty easy.
To train your body to work better at or near the LT, you will need to ride near that threshold (typically, tempo rides for extended periods (say, 20-45 minutes) very close to LT, or something like over/under intervals around the LT).
To train you aerobic system, you need to work pretty much all out, but not for very long, and not too many intervals.
I have a polar 720i, and I find it most useful keeping me from riding too fast when I'm getting in base miles.