A: You have a great question that is not easily answered. The short answer is that you do not need to go all-out hard each time you run in order to gain benefits from the workout. More than likely, if you continue this all-out approach, there is a high chance you will suffer an injury or overtraining symptoms.
A heart rate monitor is a non-invasive peek at your body's energy production system. The monitor can help estimate how your body is producing energy. This energy production and related exercise intensity can help you plan your training and recovery. Your training intensity for any given workout needs to have purpose if you hope to make improvements. Recovery workouts and rest cycles in your training help you absorb the training work, elevating your fitness to new levels.
The energy production systems most widely discussed in fitness literature include aerobic (with oxygen) and anaerobic (without the presence of oxygen) energy production. These two systems are working all the time within your body. The percentage of energy that your body produces from each system depends on your activity level or exercise intensity.
Your average exercise intensity or pace is related to the distance of your event. For example, sprinting as fast as possible across the street is different than running a 5k or a 10k event. If you want to run a couple of events averaging the fastest pace possible, this pace and average heart rate will be different for a 5k than it is for a 10k event. Know that the pace you can achieve for a given heart rate is quite trainable.
What your heart rate monitor is telling you in the question you posted, depends on your current fitness level, how much warm-up and cool down time was in your average heart rate value and the pace or intensity for each segment of your run. Based on many of the athletes I have coached, an average heart rate of 161 for 50 minutes is a relatively tough workout.
The real question is did that workout help or hurt your efforts to reach your fitness goals? There really isn't a straightforward answer to your question without knowing more information about you. The answer depends on your goals, personal training zones, pace at any given heart rate, current fitness level and how that particular workout fits into your overall training plan.
There is an entire chapter of my book Triathlon Training Basics devoted to discussing energy production and how to do your own testing to estimate training zones to get the most out of your workouts. The book also includes four detailed training plans for runners, looking to run a 5k or 10k event. The plans were intended for individuals looking to be part of a triathlon team, but the plans also work great for athletes looking to run one of those events on their own. These plans will be posted on the interactive training system, ActiveTrainer, sometime later this year.
Even if you don't use my book, I highly recommend you begin educating yourself on training concepts. There are dozens of books that discuss training theories, training periodization concepts, heart rate training (know that there is not one single methodology that all coaches and scientists agree on), pace training, nutrition and injury prevention. Hopefully, by educating yourself you can stay healthy and fit for a life time.