Someone will ask, what is the peculiarity of running uphill? I will give an analogy: you can ride a bicycle at one speed and turn the gear 36x21 and give out, for example, 150w, and keep the cadence 90 revolutions, the pulse say 140 beats, and you can go 52x11 at the same speed, but keep the cadence 20-30, that is, with each pedaling literally break the pedal, and have the same pulse 140, but... in the latter case, You load the muscles differently, and you can pick up this transfer, You will not be able to eventually push through, You won't have enough leg strength, and the pulse will still be low at the same time. On a flat ground, the muscle can not be loaded so much as running uphill. Running uphill or running with a decent gradient is very good for training type iib muscle fibers, and the heart rate can be low, thereby allowing you to run more hours a week and not get overtrained. Only you do not confuse individual work in the mountains like hills strides with a gradient of 3-7%, yes, these are good exercises, but they are unfortunately not enough, since we only perform them for a couple of minutes a week, when we need hours. Your entire weekly run should be on a gradient.