Walk/ running is fine. Its also helps avoid injury by doing to much, too quickly. If you can get out of the gym, its much nicer running outside (even in towns but its better for your legs on trails). It does feel different from the treadmill. You learn to cope with wind, rain and heat and hills(I hate hills). If you live in an area with lamposts use them as goal posts, walk between two, run between the next two, gradually increasing time and distance runnning. If you can find someone to run with. In the UK they is a women's running network with beginners groups , maybe there is something like that near you.
September might be a bit soon to be running 5k all the way but don't let that stop you. In most races it doesn't matter how you get round but as a slow run/walker myself I look at previous years results to make sure that its not a race full of speedy ahtletes who will leave me standing.
Lots of good advice on making the transition from walking to run walking on Jeff Galloways site
http://http://jeffgalloway.com/training/beginners.html
the section on walk breaks is also useful.
Good luck, be prepared to find that running can be addictive
