Complex sentence: It took New Horizons 10 years to reach Pluto-which is three billion miles from us-meaning it would be about 26 million years before it reached Kepler-452b, clearly an unthinkable amount of time.
Split into simpler sentences: It took New Horizons approximately 10 years to reach Pluto-which is three billion miles from Earth. At this rate, it would take the craft about 26 million years before it reached Kepler-452b, clearly an unthinkable amount of time.