It was my custom to sit out near the kitchen door and read t

这实际上是个it 引导的强调句型,it 为形式主语,实际主语为to sit out near the kitchen door and read the newspaper
原句语序为to sit out near the kitchen door and read the newspaper was my custom.
为避免头重脚轻,才用it
to sit ...and (to) read...都是不定式,不定式可以充当名词,必须用动词原形,and 前后动词形式要保持一致

it's a custom to do sth 习俗或生活习惯