When I started out, that's about the speed I averaged when riding on a relatively flat path (towpath in my case). Too challenging to quantify a monthly goal. But maybe just take the average miles you're riding now.. decide how many times per week you're going to ride and multiply that out. The bonus is, if you keep riding 3+ times per week your speed will increase, so you'll be doing that many more miles in the same time period (say if you get up to 10mph).

In all the reading I have done on building mileage is to add no more than 10% per week with riding 3 or 4 times a week. I start the spring with 3 rides, even though I am on my trainer all winter 5 days a week. I have three rides about the same distance, then the forth ride is my long ride that I add miles to. I did 35 miles last week and then 56 miles this past week. it was too big a jump.

Instead of speed, I work on cadence. Yesterday, I tried to have a cadence of 90 the entire ride. I usually pedal around 75, but read that higher allows for longer rides. Your speed will be determined by your bike. I have three road bikes. On one I average about 10 mph, on the next it is about 14 and on the final is about 17. I would ride my fast bike and my wife would ride her bike. She could never keep up. Last year, she road our daughter's bike, which is more like mine, and she could beat me up every hill. Needless to say, the bike make the difference.

After years of listening to others, two years ago, I saved and bought a garmin. It allows me to look at the more important issues like heart rate. I watch my heart rate when I climb big hills. And, on the flats, I work to keep it up.