It is never too soon to start the first phase of your training. You need a foundation to build on. Whether your goal is to eventually run a marathon, or you are just content with getting through your first 5K, building your base (or foundation)is your first step. What I mean by "base building" is simply this, you need to increase the amount of time and/or distance you can spend moving consistently at a brisk pace.
The most tempting part during this phase is to try to go out farther or faster than your body is ready to go, especially if you were ever very active and expect your body to be able to do what it used to do. This is probably the main reason people rarely make it past this phase--it is not enjoyable. However, if you will just start by walking, or a very slow jog if you are able, you are much more likely to enjoy the experience.
I suggest starting somewhere where there is a good view. There are many types of trails: around a park, around a lake, in the woods...with a little research I'm sure you can find one in your area. As you build your base, you may be able to walk from your house to a trail. Mixing up the scenery and terrain can help make the experience more enjoyable. But this phase of your training is not to impress anyone. Never mind the sprinters who pass by, they have most likely been doing this for a whole lot longer than you. Just keep a pace that is challenging but fairly comfortable (walking around the grocery store pushing a shopping cart does not count!). Following is a 3 week schedule I recommend base building for beginners:
Simple, 3-week base building plan for beginners
By Saturday of week 3, you still shouldn't expect to cover a distance of more than about 2 miles in 30 minutes. If this is too easy for you, you can start picking up the pace. If it is difficult for you, slow down a bit and keep that pace for another week or two and it should get easier.