I understand that one has to run fast enough to get their heartrate at a sufficiently high level to improve cardiovascular and lung health, but at what point does running faster give diminishing returns?
For example, and I'm making figures up, suppose one sees a 100% increase in cardiovascular and lung health going from 2.5mph to 5mph. Will they see another 100% return from 5mph to 10mph, or would they, say, only see an increase of 15%?
It is hardly possible to predict this with any kind of accuracy, but athletic improvements always happen in a curve that flattens out as you get better (i.e. a logarithmic curve). It must be this way, because if it weren't, you would eventually progress infinitely.
What is pretty clear is that it is not the pace that is relevant here, but the heart rate. My understanding is that general adaptions for oxygen to be taking up better by the lungs, and being processed by your cells more efficiently occurs if you run at 60% of your max HR or faster, and importantly it does not matter if you run faster. So no matter if you run 1 hour at 60% or 1 hour at 70% or 80%, these particular improvements will be roughly the same (of course other processes may benefit more from a higher HR, like improving your lactate threshold or VO2max). The reason seems to be that the stroke volume of your heart maxes out at that 60% rate - above that it will just beat faster, but not stronger; so the training effect is maximal at any effort >= 60%.
If this is your main interest in running, then running as long as you can (without injury/overtraining) at easy pace will do that for you. 60% is "easy" or conversational; i.e. comfortable, easily to chat while you do it.
Running faster trains your lactate threshold (80%-90% of max HR), "comfortably hard". This is used to train the capability of your cover to process lactic acid quick enough that you can keep up with production to avoid accumulating large amounts of it in your blood. If you run faster and thus produce lactic acid quicker than you can process it, you will have to stop eventually.
Finally, if you train even harder, with an even higher HR (i.e., interval training, fartleks etc.), you primarily train your O2 processes; i.e. the overall speed at which your body takes up and processes O2.
Of course, heart rate and pace are not related in a linear fashion either. When you run slow, increasing your HR by, say 10 points, is easy; but if you already are running fast, increasing it by 10 points is very hard or even impossible as you approach your maximum.
Opinion:
As someone mentioned in the comments, whether all of this can be considered "healthy" may be a matter of discussion or opinion, or of interpreting the statistics. Do you only look at mortality when defining health? Do you include the quality of your life as well as its duration, enjoying a strong body when getting old? Do you include the short term effects of the sport (may they be positive or negative)? I have not seen a study which includes all those effects of running or other sports - they usually simply compare mortality rates, and it is not always clear which way the causation goes.
My opinion on that would be that the benefit from easy running (a heart muscle stronger than that of a couch potatoe) probably can be considered generally healthy for everybody. The other two aspects (lactate resistance and maximum O2 processing) are probably more interesting for their athletic aspects, because those states (lactic acid overproduction or being limited by O2) usually do not occur in every day life unless you are exercising endurance training anyways.
