Yeah, I'd actually say go out for the total amount of time you'd like to run for, and just alternate running and walking. So maybe walk for four minutes, run for one minute, repeat another four times. Over time, increase the run, and decrease the walk break, until it's continuous. Walking is great exercise, but you only train those running muscles for running by actually running.
For those worried that such a “run” wouldn’t really count, plenty of ultramarathon runners use a run-walk strategy to great success. Not that I mean to suggest you’re only a “real runner” if your aim is to someday run an ultramarathon — ultramarathons are stupid, and trust me, I know! — but I don’t know any ultra runners who’d say they weren’t a runner just because their race involved some walking :)