This was on my local news last night. Interesting. They interviewed a personal trainer, it was suggested that more important than extra minutes spent exercising is to get some higher intensity work into your routine if you are able. Suggested a few exercises, like combining squats with overhead barbell presses, and also did a burpee type exercise going from standing to a crouch, extending legs, doing a pushup, then back up. So working multiple areas of the body at once, utilizing time available as best possible for maximum benefit. But...realistically we just need to do what we're able to do IMO, anything is better than nothing, increase it as we're able to do so.