Not bad advice. If you can do 3 pushups today, do it. Then do 4 tomorrow. Then 5. and so on. Or run for 5 minutes, then 6, then 7. Whatever you are able to do, add a little bit to it the next time. Not so much that it's a barrier. Just a bit more than you did last time. If you can stick with it, in a year, you'll be amazed where you are.