Drawing your belly button in doesn't recruit more "abs," doesn't create more stability and in fact is counter productive to stability. Just try to dumbbell curl with belly drawn in vs a brace.
There is no exercise that will create that 6 pack. Everyone has the 6 pack under a layer of fat. Including me! LOL. That type of leanness is all diet and some genetics. (perhaps some youth)
Running is not the only way you can keep the weight off.
If you have been stretching the same muscles for years and yet they still are always tight, guess what, it ain't working, so why do you want another stretch to do.
You have to move if you want to be healthy. Literally move.
You don't need to feel sore or like you died to have a good training session.
Sleep is important. Very important. Skimp on sleep and eventually it will catch up with you somehow.