As a few other posters have suggested, there are two really important things here - losing weight and exercise. Both are important because diet is the main factor that will help you lose weight, whilst exercise has significant health benefits even if it doesn't result in any weight loss, so it's fantastic that you've started walking and are keen to increase your exercise :)
I would recommend checking out Second Nature (previously called Our Path). It's a plan which some NHS GPs are able to refer to, but you can also sign up privately which I did and I haven't looked back!
It's a 12 week sustainable holistic lifestyle programme rather than a diet. The 'diet' is a relatively low carb diet but mostly just involves meal planning, cutting our processed junk food and when you do have a treat, eating it mindfully and enjoying it and then moving on - no beating yourself up for "failing" and then falling off the wagon! But you also have access to a registered dietician and learn through daily articles all about food, exercise, the mind, stress and sleep and the relationships between them all.
I have also read the amazing "the four pillar plan" by Dr Rangan Chaterjee which is similarly holistic in its approach, looking at "relax, move, eat and sleep".
www.amazon.co.uk/dp/0241303559/ref=cm_sw_r_apa_i_bp11EbCY954NE
I know you asked mostly about exercise, but I just thought the above might be helpful because they both address exercise as part of their holistic approach, and making little adjustments in a few of these areas can really increase the overall benefits you see. Good luck!