The most important thing is to make sure your running shoes are
right for you. Get a gait analysis done at a decent running shop. Then it is a case of combining running and walking, and slowly increasing the proportion of running, and the overall distance. To begin with run every other day, to give yourself time to recover. Make sure you stretch after each session. You might also want to consider doing something else on your rest days - swimming?
I also wouldn't worry, at this stage about the half marathon. It took me about 14months to go from complete novice to my first half. I would target a parkrun : free weekly timed 5k runs all over the country. The Hanley park one looks best for you http://www.parkrun.org.uk/hanley/course . You need to register first and print out a barcode. I'm personally not a club joiner, but parkrun is a fantastic non-threatening environment, and a great way to assess your improvements.
The best thing about starting from scratch is the incredible changes that can come very quickly. Within a month you will be running further, will have lost weight, be more toned, and will feel better. Then the bug will have bitten!
Could you give more details about the discomfort?
Good luck!