Ah, to be young again in physical terms. That may be a good image to contemplate, but it’s not going to happen. Is it? We have to be the best that we can be— given the physical condition we’re in -- at every stage of our lives. Yeah, it’s wishful thinking to envision what we did at age 25—or even 35 or 45. Yet, the real issue is this: What am I capable of today? AND do I exercise as much as I can each week? We can’t ever give up on exercising. It’s too important.
This is what I wrote in 2018: I am happy when I can do my current exercise routine. Yet, some days are better than others with regard to how I feel. I miss days when I feel weak, get bronchitis, etc. But I will NEVER stop exercising.
Here’s what I did then: [More later on my exercise in 2020.]
There were/are many 50+ year-olds at my gym. We helped reinforce each other. We all wanted to live as long as we can and as well as we can. Try it. You’ll like. 
Exercise is vital at any age and in any physical condition. Consider the following observations from various experts.
"Exercise is good for you.” Yet most young adults don't get the recommended amount of physical activity of at least 30 minutesfivedaysaweek. Your physicalactivity cantakemanyforms,fromwashing a car or going on a walk to playing Frisbee or practicingfora sport. Staying active helps you stay physically healthy and emotionally balanced. Exercise also helps you relax and burnthe calories you take in. [Palo Alto Medical Foundation] 24
Physical fitness in middle age can be a powerful protectoragainst frailty, heartconditions,and more. In fact, regular midlife exercise might be the most powerful way to prevent chronic illness, anew study done at UT Southwestern Medical Center and the Cooper Institute shows. Researchers examined 18,000+ participants with an average age of 49 and found that the more fit men and women were, the lower their chances of developing serious health issues over 26 years of follow-up. Need more reasons? Fitness can help stop osteoporosis. 25 [Beth Orenstein for Everyday Health]
Work slowly back if you are used to high-intensity exercises. For a cold, a 1-2ish week buffer between getting back to full exercise is likely good. For more severe illnesses such as influenza or pneumonia, I would take at least 2-3 weeks after all of symptoms have subsided to work back into things with full intensity. The problem with going back to high intensity right away is that even if all of symptoms have gone away, there are still bacteria or viral loads in your body, just not enough to make you symptomatic. So high intensity exercise can depress your immune system enough to make the illness come back, sometimes even stronger than ever. Thus, it is best to be conservative with this. If you’re using a typical 3x a week type of exercise schedule, start with 20-30% of typical full workouts, and ramp up by 10% for 2-3 weeks until you reach 100%. Be conservative rather than get ill again and be out another couple of weeks . 26 [Steven Low]
Have you given up on exercise? A lot of older people do. Just one out of four people between the ages of 65 and 74 exercises regularly. Many people assume that they're too out-of-shape, or sick, or tired, or just plain old to exercise. They're wrong. “Exercise is almost always good for people of any age,” says Chhanda Dutta, PhD, chief of the Clinical Gerontology Branch at the National Institute on Aging. Exercise can help make you stronger, prevent bone loss, improve balance and coordination, lift your mood, boost your memory, and ease the symptoms of many chronic conditions. 27 [R. Morgan Griffin for WebMD]