Headlong describes something done headfirst, rashly, or really quickly. If you’re the star batter on your baseball team, it’s expected that you'll make a headlong dive for the base.
Headlong can refer to hasty actions, but it can also be used more figuratively to describe decisions are reckless or made in hurry. Your sister may tend to weigh all the options, but you make headlong decisions. When you decided headlong to move to Florida for a chance to play professionally, your sister thought you’d gone off your rocker — but the day you sign a major contract, she’ll be singing a different tune.