If you're out of shape right now, I wouldn't focus too much on speed right now. Instead, focus on building a solid aerobic base. Try to stay well within a comfortable aerobic zone, i.e., not anaerobic. If you can't maintain a conversation without getting overly breathless, bring it down a notch or two. If you're using a heartrate monitor, stay within 70 percent of max. I would recommend focusing on this for a couple of months. After that, you can start building in some speed and hill work and allowing yourself to go anaerobic. I know it probably sounds sort of boring and tedious, but your body (and cardiovascular system) will love you for it.

I spent a winter doing just this when I first started riding. Boy, did I hit the ground running come spring.