Even with a busy and hectic lifestyle, most people should be able to find 20-30 minutes a day to exercise. If not everyday, at least 3-4 times a week. The best way to increase your stamina and strength in the shortest amount of time is probably doing some type of high-intensity interval training (HIIT) with weights and bodyweight. The great thing about strength training with intensity and short rest periods between intervals is that it promotes fat loss, even long after you're done working out. You achieve a lot more in a very short amount of time, it requires very little equipment and you can do it in the privacy of your own home. There are tons of videos online, but if you're in need of guidance, there are lots of great home fitness programs at http://betterhealthierbody.com that would help you achieve your goals. It still cost less time and money than a gym membership and is more effective than doing hours of useless cardio.