If I were to do it, it would take me twice as much time.
If I had to do it, I would be spending twice as much time as you.
ご質問ありがとうございます。
① "It I were to do it,"＝「もし私がそれをするのであれば」
"it would take me twice as much time."＝「私は二倍時間がかかるだろう。」
② "If I had to do it,"＝「もしそれを私がしなければいけなかったら」
"I would be spending twice as much time as you."＝「私はあなたより二倍時間をかけているだろう。」
ご参考に。
アメリカ合衆国 
