OP, you do need to take this behaviour far more seriously than you are doing (or sound like you are). If for no other reason than the dog's sake - she is one chase away from finding herself on the wrong side of the law and, with her breed in mind, people will not give her a second chance.
All dogs should be under control at all times for the sake of other people but breeds such as Rotties need to be visibly under control at all times for their own sake as well. People will not allow them leeway and you need to be aware of that and behave accordingly.
You also need to understand that chasing and barking is likely to be an aggressive behaviour. Possibly not one likely to lead to a bite (individual dogs vary) but it is likely she is barking and chasing because runners make her feel nervous and so using her behaviour to warn them off. That's aggression - and even the sweetest dogs can display it. It's not a measure of how good or bad she is. But she is trying hard to communicate and you need to listen.
Please put her on a lead at all times unless you have clear line of sight in all directions and know you are not going to be surprised by anyone. Personally, even with several years of dog behaviour study now under my belt, I would not have this dog off lead in any public setting for the foreseeable.
You can then work on desensitisation. Work at a distance SHE feels comfortable - which basically means, if she reacts then you are too close to the runner and you need to be further away. Be clear on this: for many people they struggle to understand the dog needs 50m, or even 100m of distance. But if that's what she needs, that's what she needs.
Only allow less distance to the next runner if you have succesfully had several occasions of non reaction. If she ever reacts, go back a stage and have more distance next time.
It can help to use treats to partner desensitisation with counter conditioning. If she looks at the runner and then looks away (e.g. to you) without reaction, mark the behaviour with something like short praise and treat her. The idea is that, over time, she finds it rewarding not to stare/focus on the runner but instead to focus on something else. Focussing on you is an especially good option. Be preppared for this to take 100s of repeitions over several months to get good progress. Consistency is key to this - there are no short cuts.
Please take this seriously. It can be hard to hear that people find your dog threatening when you know her to be a sweet dog with you. But you must put your ego aside and do what is best for her - you must keep her safe by visibly keeping everyone else safe.