Some people say 10% increases in distance, but I wouldn't overthink it. Just pick someplace that you'd like to go that's a little further from home each time. I think it's more entertaining if you've got a goal destination that you're trying to get to. When my husband started riding with me, our first ride was just downtown for breakfast, then the next time it was to a park we wanted to check out, then the next time was to a reservoir a little bit out of town, etc. Before he knew it, he was doing 50 mile rides without feeling like he was killing himself to do it.