She's really casual about having had serious battles with other magical girls - somehow I feel like both those scars and her obsession with becoming physically stronger might be linked to her sister being known as "the strongest magical girl" (at least according to the rumors that Seiko and Joshua heard).
I wonder if there were some magical girls who considered taking over Sakura's turf, or to just collect on some easy pickings now that she wasn't around anymore to protect the Darkness clan members she protected.
Momo mentioned early on that in terms of magic, she was never a particularly strong magical girl to begin with. So training her body might have been her only option to survive a situation like that, until she had grown a reputation of her own.