HDL, unless genetic, is a product of poor diet and exercise.
Insulin sensitivity is predominantly a product of too much adipose tissue aka FAT. Belly fat is the worst.
Increased blood flow can be accomplished by high intensity resistance training.
My point is, the OP needs to build muscle (especially considering his age and comment about the difficulty he is now experiencing with building muscle. Hint: anabolic resistance and sarcopenia) and for optimal results he would be wise to avoid activities that hinder that.
I am not against cardio... it's just not the right tool to pull out of your toolbox to work on his problem.