Complex sentence: Lee used the month-long pause in McClellan's advance to fortify the defenses of Richmond and extended the works south of the James River to a point below Petersburg; the total length of the new defensive line was about 30 miles (50 km).
Split into simpler sentences: Lee used the month-long pause in McClellan's advance to fortify the defenses of Richmond. The total length of the new defensive line was about 30 miles (50 km).