The basic Training Peaks account is free and you would be able to analyze the first and second halves of your runs that way. If you are running on rolling or hilly terrain you will not be able to nail down your AeT. Please watch the AeT video as it explains all this.
If you are trying to build aerobic base with these runs it is not the average HR you care about. It is the maximum HR you care about. You want to keep the HR below the AeT to develop more aerobic capacity.