Someone asked this over there, it seems very pertinent and an official (or correct!) answer would be appreciated. I'll quote their entire question:
"Does Sprint X let a hero move X spaces in addition to its two actions, or does it add spaces onto a travel action? Could a character Sprint and then attack twice?
The rules seem to imply yes, but do not clarify...
Sprint is a keyword that allows a hero to move extra spaces.
89.1 When a hero is taking a turn during the action phase, that hero can discard a card that has the “Sprint X” keyword to move X additional spaces.
89.2 A hero can perform other actions and resolve other game effects between each movement that is provided by the “Sprint X” keyword.
89.3 A hero can only discard a card to use the “Sprint X” keyword if that card is prepared."