Run could be easily compared to bodybuilding in terms of subject complexity (FITT principles) and wide areas of specialty (long/short distance, speed, terrain, combining etc.). To even make it worst, we live in information overload era. Hopefully, further reading below will shed some light on effective approach to constant improvements in your run performance while staying effective.
Any training process to be effective must be planned, long-term executed, scientifically based and having optimal condition. You should definitely have a goal in mind and follow SMART principles and figure out your FITT (frequency, intensity, type and time). It is probably the best idea to always advice your doctor before throwing yourself into any physical activity. There are some training myths and laws that are not true and could be broken.
Actions matter the most. Once starting your run session, you could do fast-walk and stay consistent and slightly improving your performance. That way, you will constantly progress and prepare your body and mid for more strenuous workouts. Consistency and right approach is more than frequency. Two runs a week will have tremendous benefits on your health and you can still top your own performance even when training twice a week. Now you wonder, what to do when going for competitions and striving for top performance.
Since have been working on complete athlete project and been through couple of competitions, one could say that combining right elements is king. Each skill and training gets less challenging with time so even complex tasks starts to seem manageable. Meaning, combining more various training aspects (variations or training cycles) in your run unit with time (patience for adaptation) will get you more progressive while remaining effective (weighted vests, intensity, terrain etc.). Basically, you make two runs more complex so you remain progressive while effective.
This is one way around crashing your run training. Adding more complexity to one single training unit is going to stimulate new limits to your capacities. The other way is to combine that one training unit with another training unit of different nature that you are going to benefit from. As we pursue the efficient solutions, we are not going to run 3-5 times a week, except you are rich and have lots of free time and share lots of enthusiasm for it.
The most effective way of constant improvements in your run training is to start hitting gym efficiently. I strictly believe that, whatever goal in life related to health and performance must be reached through exercising. It is the only way to prevent, postpone and decrease the process of getting weaker and fragile with age. It also assess the quality of your life. Caution It is more the food that matters the most in scientific way but hopefully you got the point. Exercising makes you stronger and last longer with the right food, though.
Having one or two fitness workouts a week while running two times a week amounting to 3-4 train sessions a week seems like very productive kind of training process. Imagine what could you achieve with that type of training schedules while staying progressive. You could become extremely strong overall with extreme endurance which could make you a great athlete, although not a complete one.
Due to complexity of the subject and wide specialty it requires closer monitoring and definitely more posts on different aspects which you will get updated on. To sum up all, you can always use and benefit from what you learnt in previous projects and turn it to a new ones.
Training twice a week is enough when it craves strength and endurance. Then, it is just your actions that determine your progresses. Combining effective methods in one training unit (run-related) with another training unit of different nature (fitness-related) supporting your overall goal (sprints/endurance run) are proven to be mutually beneficial while staying effective.
The most important question remains how much of what specifically to stay in homeostasis (balance) with your goals. Being progressive creates progressive working capacity while one must choose wisely where and how much of this energy is going to be distributed as it is limited.
Some interesting remarks to consider before throwing yourself fully into the sport. Too much muscles could get you a heat stroke and you are never going to have same results as marathon runners when being bodybuilder, neither he will have your results. Endurance sports prolongs longevity by 8 years which is great additional benefit from run. So, one needs to make sure where his/her potential lies the most and where to put most of the energy. (F.H. Acker 2018) (G.E.Clénin 2016)