Their are too many variables. Heat, recoovery, hydration, it's far too complicated to base your training on that alone. Take a high heart rate into consideration but slowing to a walk just so you can keep it low is over the top.

Emotional stress, coming down with a cold, illness, etc will kick it up also. Walking is not a sin. Matter of fact, most folks that have been at this for over 3 decades usually have a little walking in their programs. I know folks who run/walk marathons and finish faster and look refreshed at the end of a marathon as compared to the folks who run for 18-20 miles and then do a death march to the finish. Worse yet--those death marchers normally don't run any more marathons.

Relax--walk some--make running fun. Very few on these boards will ever win an Olympic Medal. I don't take myself too seriously and have amassed over 64,000 miles of basically injury free running over the last 31 yrs and attribute a lot of my success to taking it EZ--on 95%+ of my runs.
If you want to be a lifetime runner--slow down--have goals to run the rest of your life. If you do, there is no hurry. Got it? Nick