Getting strong(er) is possible. It is a simple process that requires hard work and consistency. Whatever genetics or baggage of poor health or past history of bad habits you might have, you can get stronger. Yes. You can.
Stamina is an even simpler problem than strength.
Flexibility, likewise. Doable. Simple.
Stamina and flexibility are simpler because the barrier for entry is lower than strength training. Spend 30 minutes a day and in 3 months, you will be a million miles ahead of where you are.
Start small. Start today. Don't waste time figuring out the best method. Pick something. Do it daily for 30 minutes. Don't second guess yourself.
Don't pick something that yells fad.
Walking or running works for building endurance. 30 minutes - how much distance can you cover? Now, every day, try and cover a bit more. When you are stuck, increase the time to 40 minutes. Repeat this until you are at 60 minutes and 3 months.
Yoga works for flexibility. You can find a 30-minute routine on YouTube. Or just the wonderful surya namaskaram is ample. Do it for 30 minutes. Daily. 3 months.
30 minutes/day for 3 months is what lies between the old you and not being the old you.
Find 30 minutes. Take control.