1)
Why does it say "Birkai Kashlu mi'Tzom"?
1.
Radak: He fasted when he fled, so Hashem would have mercy on him. David became weak, and did not have strength to go.
2.
Sha'arei Teshuvah (4:12): A Bal Teshuvah should put, in place of Aveiros, afflictions, fasts, tears, and abstention from pleasures.
3.
Malbim: You should not punish me for my sin, for my legs are weak from fasting and my flesh is lean from fasting and Teshuvah.
2)
Why does it say "Kachash mi'Shamen"?
1.
Radak: I am thinner than the fat that used to be in my flesh. Now my flesh is lean, from fasting and fleeing.
