Hi,
I have now been doing Ashtanga for about five years and I still find it both mentally and physically demanding practice. However, it does get easier. I too was extremely inflexible when I started, but over time your flexibility improves.
John Scott is an excellent teacher and has produced a very good DVD. Try not to feel over whelmed with it. It really does not matter how flexible you are or how far you can go into a pose. I am sure that over time you will find it easier.
Although DVD’s are a great way to learn, there is no substitute to going along to a class. So if you have time and the means to do so, I would really recommend you going along to one regulary. There you will experience the expertise of the teacher, who will provide necessary corrections to your posture and encourage you to go deeper into your practice without risk of injury. They will also provide you with alternatives if a pose really is too strong to start with. You also have the bonus of meeting other people, who probably feel as you do.
If learning at home is your only means, then I would just encourage to stick at it. Enjoy your practice and take it gently, I am sure that in time you will be surprised how flexible you have become, and don’t be too hard on yourself. If necessary use props to help you (blocks and a straps), these will enable you to get into positions without applying too much strain on your body and thereby allowing your yoga practise to evolve.
I hope that helps
Darren 