congratulations on getting back on track with an exercise program. What I read is that you are doing weight training 4 days a week in a split routine, one day cardio and (I assume) yoga for flexibility.
What strikes me about your program is that you seem to limit your cardiovascular exercises. If your goal is to improve stamina (which I read as cardiovascular endurance) then cardio once a week is not enough. I would suggest to add some of it on to the weight training days, maybe as an extended warm-up. I realize that time can be a factor, and you may need to modify your weight training.
You do not specify what you do on your weight training days. Current thinking moves more and more towards whole body training rather than the ‘old school’ of weight machines or even dumbbells in linear movements. By its nature, core training is a part of this way of training. If you do not do any of that, I would suggest to educate yourself about it or possibly get a trainer to help you with it.
I wish you good luck in your pursuit of health and fitness.