Hi mumofone, don't be too hard on yourself. 40 mins walk is a good start. Walking is an excellent form of exercise. I walk during my lunch break every week day for 30-50 mins. Admittedly I do other exercise as well now but if I don't get to anything else I still count that walk towards getting healthier and fitter.
Also if you haven't exercised for awhile you need to build it up slowly. Maybe set mini goals like increasing the walk by 10% each week or setting a kilometre challenge. There is one of these going in the challenge forum already. Maybe find a walking partner or get some exercise DVD's/videos to add variety to the exercise you are doing.
I know, I know easier said than done. However, don't just 'want' to do it, choose to do it. Make it a choice rather than a chore. Geez I'm sounding corny now!!