First up, IMO running is a great way to train (but 6 miles is not long distance).
All boxers need endurance in their legs so there is nothing wrong with roadwork and you're right that many past champions ran for miles.
However, it is a good idea to mix things up and vary your training. Interval training for instance mimics the unique requirements of a fight - the intensity is high throughout but with peaks and troughs. This gets the body accustomed to periods of exertion and periods of lower intensity to recuperate.
Also, distance running does not involve the arms as much as enhanced interval training (with exercises like push ups and pull ups that involve the entire body).
Long distance running takes time (pro fighters have more time to train and recover). A short high intensity interval sessions takes less time yet will elicit a similar adaption in the body in terms of aerobic endurance (with the added benefit of anaerobic improvements too).
Less time devoted to conditioning means more time for sports specific training and skill sessions.
I enjoy running but it places stress on my knees and this gave me patellar tendonitis. Personally, I find interval training with short sprints, burpees and calistentics better for me.
Ultimately, it's up to you. If you enjoy running and you have time then do it.
You can't really look at other people, especially pro athletes, to find the right way for you. Like Ross has said countless times, some fighters run and some don't. Some fighters lift weights and others don't.
Just be sure that you don't run too much and leave yourself drained.