When you are chased by a bear your body goes into a fight or flight mode. I’d recommend flight. It doesn't matter to your body; your body will call upon all its resources to survive the encounter. Your heart rate increases, your heart pumping volume increases, and your venous return increases as your body makes an all-out attempt to survive. If you survive your body will make a positive adaptation so that you are better able to survive the next encounter. From a cardiovascular fitness standpoint it would be a good idea to be chased by a bear from time to time. Barring that one could do some form of high intensity training (HIT).
As you reach flight or fight, either encountering a bear or a HIT session on a bike or in the weight room, there will be an increase in endorphins to deal with the pain caused by the exertion, and your adrenaline system will be activated resulting in another positive adaptation.
From this study Scripps Florida Scientists Unravel the Molecular Secret of Short, Intense Workouts: