I will put it more bluntly------------no pain, no gain is a crock. No matter what your goal is, an intelligent plan to reach your goal should not be painful. Pain is simply your body telling you that you are damaging something.
IMO pain is for fools. That is why among other reasons I ride bents. You can ride a bent all day without pain. Sure you will get tired, but you will have no pain. I ride approx 30 miles every other day. It controls my weight, and is fun. IMO gain can come with fun and no pain.