I guess it's just a different approach. My feeling is always extend the dustance, target places furtger away. Get more miles under your belt and your natural speed will come.I would suggest the exact opposite, but I think I know what you meant. I would target rides of a certain duration, not rides of a certain distance. So target a 30 min ride, a 1hr ride, 2hr ride, etc. The actual distance covered during that time (as a beginner) is not important.
It's very rare to hear peopke on here say "I cycled for 3 hours yesterday" more likely to say " I cycled 50 miles yesterday"
Actually if someone had said I would ride 100 miles a year ago...
Im not exactly the fastest or the fittest rider there either but I can easily cruise around 16-18 Mph in the right conditions. Keep at it and you will improve!