I'm really not sure what kind of time would be considered fast for alternating between walking and running. I don't think the time matters much to be honest, because ultimately if you are sprinting to your fullest capabilities then I would think you would still be getting the same great benefits of sprinting that faster sprinters get. It's all about giving it your all, not so much speed, in my opinion.
I for one know I'm by no means a super fast runner or sprinter. The last time I ran a timed mile was right around 7 minutes, but that was a few years ago when I was on a track team, and I'm sure it would take me longer now. One of my female friends on the track team at my university can run a mile in a little less than 5 minutes, which is just insane to me. I don't go for a run very often now, because I don't enjoy it much anymore, but when I sprint I give it all of my effort, and I think that's what really matters.