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。
If it were me, I think it would take me twice as long as you.
I think you are twice as fast as I would be.
この場合、次のような言い方ができますよ。
ーIf it were me, I think it would take me twice as long as you.
「私だったら、あなたの２倍時間がかかると思う」
to take A twice as long as B で「AだとBの２倍の時間がかかる」
ーI think you are twice as fast as I would be.
「あなたは私がやるより２倍早いと思う」
このように逆の言い方もできます。
ご参考まで！
アメリカ合衆国 
カナダ 
