I know a woman who was seeking a certain job. She was told that in order to get that job, she had to be able to run four miles. She formulated a plan.
She measured off a route that took her three miles from a certain parking lot. Each day, she would drive there, park her car and run as far as she could comfortably run along that route. When she was winded, she would drop back to a walk, but keep on the route. Once she caught her breath, she would run some more. She didn't push herself, but would run/walk until she reached the three mile mark, then she would walk the three miles back to her car. At first she walked more than she ran, but that changed a little each day. In a few weeks, she only had to walk on the uphill legs. A couple more weeks, and she was running the whole way. Then she started jogging back to her car. In just over two months, she could jog (not too fast) the entire six miles, to her mark and back, without dropping to a walk. She never became terribly sore, and soon noticed that her jeans were tight in the thighs and very loose in the waist.
She went for her physical test and learned that she only had to run a mile. She aced it and got the job.