First off, please don't compare yourself with the miles I do because I have been cycling for almost 30 years. If I can ride at least 100 miles a week, I'm happy, but lately I have been doing almost 200 miles per week. I am preparing for a 200K brevet in the middle of March, but it's not always the miles you need to do, but the quality of the workout. I manage to average at least 15 mph on my solo rides and with others, average 16.5. You can always go faster in a group and make the ride go a lot faster, too.
There's a group of riders, who are in their 70's and 80's. They don't ride very fast, and when they stop, they will take a 2 hour long break at stores. They will probably cover 85 to 90 miles on a Saturday, but none of them are knocking themselves out. Me, I will go out for a ride, and I'm done for the rest of the day. Like I say, it's the effort you put into your riding that makes you stronger.
You can also go out, once or twice a week and do speed intervals. Pick a road that's not too traveled and go out and ride hard for a quarter mile, one mile, or just count for 30 seconds. To start, do 4 intervals and gradually increase them to however many you want. Then go home. You may not have done a lot of miles, but you have put maximum effort into your training. You would be surprised how much faster you will become, and the stamina it will build.
