Hey, i started by using the walking/jogging method about 6 weeks ago when i`d been doing strength training, but my cardio was non-existant - so i was really pretty unfit.
There`s no doubt you CAN do the 5k and be ready - when i started (i still can`t "run" because my right knee would collapse), i couldn`t "jog" for more than about 30 yards, and today i did my 2nd 10k in 2 weeks, and i`m still not that fit! (first one was 1 hour 45 (and only 9.7 k really,), 2nd one was 1 hour 34 and 10.1k!)
I`m gonna enter the 10k great Scottish run in October, and i still consider myself relatively unfit!!!
(as for walking, even some marathon runners resort to walking - i`m gonna for some of it!)