Hit the ground running
Get off to a brisk and successful start.
'Hit the ground running' didn't originate in WWII, as is often reported. The literal use of this phrase saw the light of day sometime toward the end of the 19th century in the USA. An early citation of it is found in a whimsical story which was syndicated in several newspapers, including The Evening News, 23rd April 1895, in a piece headed 'King Of All The Liars' (and should their readers have not got the picture from the text, they were kind enough to provide one):
"I turned to run and figured to a dot when he shot. As he cracked loose I jumped way up in the air and did a split, just like what these show gals does, only mine wasn't on the ground by six foot. The bullet went under me. I knew he had five more cartridges, so I hit the ground running and squatted low down when his gun barked a second time."
There are many references to the term in the early 20th century. These all use hit the ground running in its literal sense and relate to the various ways people might do this, for instance, hobos jumping from freight trains, troops being dropped by parachute etc. The first figurative use that I've found so far, that is, a usage where no actual ground or running is involved, is from The Hayward Daily Review, October 1940:
"It sometimes seems to me that the young idea nowadays wants to hit the ground running and to tell the old editors how to run things."