Don't be discouraged. If all you can manage today is to resolve to get fit, that is still better than throwing in the towel.
You would have laughed if you could have seen Paul and I after we bought our first exercise DVD several years ago.
We put it on and sat on the couch and watched it with curiosity. We could not imagine being so robust! :)
It was only the One Mile Workout, but the next day, when we tried it, we did it very gently and sweated and puffed a lot.
Trust me, we've had our fitness ups and downs, but I'm a 57 year old woman. If I slide now, who knows if I will ever get back up! You have youth on your side and I know you can do it.