I've just put some stuff on the weight loss thread but this is just as relevant for 'non-dieters'
Any of you who are really wanting to 'up' your exercise, have a look at the site Race For Life . All around the country there are women-only 'races' in aid of CancerResearchUK, it's an amazing day; thousands of women getting together to run/jog/walk (and over half do walk) for 3 miles/ 5 kilometres, many have 'I'm doing this because of X' or 'In memory of Y' labels on their backs.
I did it last year with a group of friends and now run regularly, even entering 'proper' races. It got me hooked and it certainly helps my weight maintenance but I would never have started without it. Have a look at the site and stop making excuses, even to walk 3 miles takes well under an hour - you HAVE got time.