Hamstring flexibility plays a big roll in L-sits and L-hangs. If your hamstrings are short then they will literally work against you, pulling your feet towards the ground and forcing you to bend your knees. I cannot overstate the value that is bound up in full range of motion!
20 minutes to establish 1RM front squat
Interval (pick your poison):
6x500m row, rest 2 minutes between efforts
8x400m run, rest 90 seconds between efforts