Complex sentence: Within 3 years it had pushed Jane Fonda into second place on the all-time individual best-selling fitness video chart and it remains the all-time fitness bestseller in both the US and UK.
Split into simpler sentences: Within 3 years it had pushed Jane Fonda into second place on the all-time individual best-selling fitness video chart. To this day it remains the all-time Number 1 in both the US and UK.