See if this works for you.
Do a 30 minute session and split it into 10 3 minute "cycles".
Each cycle works like this.
You walk for 2 minutes 50 seconds and jog for 10 seconds.
Do this 5 times a week for the first week. I would suggest not doing the 5 days straight, do it perhaps 3 on, one off, 2 on, one off, so you're not doing 5 days, then doing 2 of nothing back to back.
In the second week change each cycle to this.
You walk for 2 minutes 40 seconds and jog for 20 seconds.
In week 3 you change each cycle for this.
You walk for 2 minutes 30 seconds and jog for 30 seconds.
Eventually you should be able to go for 30 minutes of continuous jogging.
Use a pretty flat course to start with, until you're able to do a whole week of complete jogging, then you could change things in one of three ways.
1. Use a tougher course, eventually going for a longer route.
2. Jog for another 5 minutes per session increasing your total jogging time week by week.
3. Try to go for more distance in 30 minutes, by speeding up.
Hopefully this will work for you