If I were motivated enough to do this everyday, I would've been skinny by now.
If this works, I would've lost weight a long time ago.
If I were motivated enough to do this everyday, I would've been skinny by now.
(これを毎日やる意思があるなら、今もうすでに痩せている)
If this works, I would've lost weight a long time ago.
(これが効くなら、とっくに体重が減っている)
場合によっては、主語はIでもOKです。