I started exercising last week, and I've been walking for half an hour in the mornings (lots of hills), and going to Curves 3 days per week (which is circuit training with resistance weights).
After 2 weeks I feel like I'm not progressing enough, and I'm going to increase the walking to 45mins.
However, these things take time and it's best to start small and work your way up. Once something starts to get easy for me I make it harder, eg, by adding more hills, or walking for longer.
Hope this helps...good luck with it all!