If, like many people, you think your retirement savings aren’t on track, which do you think can have the most impact: saving more of your income or working a bit longer?

Setting aside a significant portion of your income is obviously a sound approach for building a nest egg, especially if you start saving early on. But working a little longer—an extra couple of years or even a few extra months—can have as big a payoff as saving substantially more, according to a new study.

A...