Just be aware of it. It can happen on shorter runs depending on climate and hydration status. If you time decreases and your HR remains the same, this is a good sign of improved fitness. Also, if you find you are running at a HR that once caused you to be slightly breathless and now are talking casually, that is a good sign of improved fitness. HR should return close to normal after 30 minutes or so.
Hydration is important.
One thing to track is the time to get back to 80. Also, tracking resting HR can help. A HRM is a great tool, but do not become a slave to it. One of the best uses for someone training is to hold the person back on what should be easy days. A HRM helps dial it in.