Which one is the best way to stay in shape and see physical results?

Which one is the best way to stay in shape and see physical results?
Do slow work out exercises or challenge your body? For example. Do certain type of exercises but in a slow range but consistent or challenge yourself and do different exercises in order for your body not to come to a plateau and get used to the slow exercises always.

I had heard from a fitness instructor that "Not evertything has to be done fast, slow things can alwayso work for your body and you can see results physically

But then I heard other fitness instructors who says "you have to challenge your muscles so see results, cause doing the same slow muscle movements each time, the body gets used to them and it will come to a point when the body settles, you have to do different body movements". Also anotehr related queston. Is doing just one type of workout exercise or sports discipline will really make your body change or you have to do different things so your body does not get stalled and then you dont see any results anymore? I have an acquaitance who is a running devoted fan, for her running is her life, she does not do any other kind of exercises or sports discipline, she just run. Im not denying running is not a bad sport, sure is good, but is just running a way to really make your body fit and be in great shape. This lady tells me that running is the ultimate sport, you brun calories everywhere and be fit all the time and you tone your body, she says it is not necessary to do other type of sports, or lift weights or strenght training, cause running gives you all.

Sometimes I want totell this lady that she has to do other kind of work out, do weight training, so the body does nto settle in just one type of thing, but each time she swears that running is her love and there is no other type of sport like running in this world so, she defends running 110%% to be the best sport out there, so I decide not to say anything and stay shut cause each time she will question me.
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I do have heard and see for myself that some people who do make running part of their lifestyle, some of them end up super super thin, like their muscle mass dissapears, the bladder in women I heard lower (is this dangerous?), no breast tissue anymore, flat as an ironing board. Runners do develop good calf muscle that is something I know and see for myself, cause that is the muscle on the leg they mostly use.

Which fitness instructor is right, or both are correct?
Which one is the best way to stay in shape and see physical results?
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