I totally know where you're coming from.
I used to spend 16 hour days hiking and spending time in the mountains. I felt great, and nothing seemed to be too much. Now, I have times when it's a grunt to make it from my easy chair to the bathroom. The only thing that has happened is a virus I had for about 7 months, 2-3 years ago. Ever since then it's been an uphill pull just to stay alive.
The problem is, I'm losing my battle. I've had several doctors tell me I'm on my way out, and it ***** at 56 years old, but I guess that's the way it goes.
What I've been told about exercise is only to do what you feel comfortable with. Learn to listen to your body, and if you begin to feel short of breath, or fatigued in any way, stop and rest until you no longer feel the shortness of breath, or fatigue, then try again.
I seem to get a lot of good out of weight lifting. I'm not talking about bench pressing 100+ pounds, but using 5-12 pound weights and using my arms to bring them to my chest, doing 30 reps per arm. Then I attach the weights to my ankles and do the same with my legs. You can buy weights at Walmart that use velcro to attach them around your wrists or ankles. It's a slow process, but it helps to at least make an effort in the right direction. Hope this helps.