Ying and Yang Bro!
If the hips are tight, look to the glutes. The glutes moves the legs. The hip flexors work in opposition support the glutes. Years of inactivity make your glutes shitty, so the muscles of the groin and hip flexors take over to do they job. They are not really designed to be used this way. Every time you kick, you're making your hips and groin tighter. You can stretch all you want, but every time you move improperly, you cause those muscles to get tight again.
You have got to get your lower body working correctly again.
Put a resistance band thru your lifting belt, and walk side to side like this
This will force your glutes to work to support your pelvis. Do this a bunch.
In the morning, walk around barefoot with ankle weights on. The weights are just heavy enough that you have to take each step properly. They will make you aware of where your toes, ankles, and knees are. All of that is handled by your glutes.
If wearing ankle weights sounds weird, pretend you're some kind of Ancient monk karate student taking 1000 steps in knee deep water.
For strength and endurance, you should be sled dragging. Its awesome. Attach the sled to your lifting belt, and drag it around with a power-walking, almost alternating lunge style step. Every step targets your glutes, while challenging your endurance and building your legs. Each time you drive off with your Glute, that hip flexor is stretched.