Nice job! FYI-Walking is not a warm up for legs. if you are doing a very fast paced walk you are only getting the blood flowing and not really warming up the leg muscles. You will be likely to develop an injury without proper leg warm-up. "THINK LONG TERM" I like to use the Plyo ball to warm up for leg curls and low back. Also, hyper extensions, which is a bad name. lol
Planks front and sides. I won't go into specifics as not to try to explain all the exercises. i don't trust referring people to go to youtube unless i looked at the video as i see to many errors in technique. Books of reference. Mark Verstegen "CORE PERFORMANCE" and dr. Nick Dinubile "FRAMEWORK".
Squats are number one overall leg exercise, deep in the bucket!