Good work on the running .. and keeping it off-road
A couple of thing on your current plan - it's great that you are run walking, you need to build up those muscles / tendons etc which can take longer to handle the stress than your heart and lungs. Be aware that no carbs and running can help lose weight - but can leave you feeling pretty drained at times! Also - you might be putting ON weight through building muscle, so as your running increases, you don't always see your weight dropping off on the scales (don't worry - the mirror tells you the truth).
Running on trails is great as is works a bit more of your body (all that dodging and weaving) .. so some core muscle exercises and ankle exercises can help (links below).
Good work on your long term goals with distance - but your biggest obstacle to overcome is getting there uninjured (it's not that everyone gets injured, or that it's easy to get injured, but people have a habit of getting a bit carried away and increase their mileage too quickly). 10% extra per week, like baldy says.. which sounds boring, but gets you where you want to go safely and without a month on the sofa falling out of the habit! A 10K is the good next step and you'll have a better idea of how your body is reacting to increased miles
You can find a lot of training plans at Runners World (.com or .co.uk.. maybe there's an SA one too). You can look at plans which are for a specific distance or that match the number of runs you do per week. But don't pick a plan because you like the race finish time and ignore the fact that it increases your distance by 50% in the 1st week!
Here's some training plans from Runners World
5K - http://www.runnersworld.com/article/0,7 ... -0,00.html
10K - http://www.runnersworld.com/article/0,7 ... -0,00.html
Article about trail running exerciseshttp://strengthrunning.com/2010/05/trai ... -training/http://www.runnersworld.com/article/0,7 ... -0,00.html