1)
What increase does he request?
1.
Rashi: If it was decreed that I die young, add days until my years will be 70, "Kemo Dor va'Dor" (like a standard lifetime - "Yemei Shenoseinu Shiv'im Shanah" (Tehilim 90:10)).
2.
Radak #1: This refers to David, after he was anointed be king. He requests that Hashem add years to his life, like "v'Hosafti Al Yamecha" (Melachim II, 20:6).
3.
Radak #2: This refers to Galus. The king is Mashi'ach.
4.
Malbim: If You will increase each year allotted for me to be like a Dor, i.e. standard lifetime of 70 years
2)
What is "Kemo Dor va'Dor"?
1.
Refer to 61:7:1:1,3.
2.
Radak: It is two Doros (twice a standard lifetime, i.e. 140 years).
