If you are planning on riding in a 4,200 mile bike race over about 16 1/2 days...
Or you are planning on riding in a grand tour with 21 days, and stages from around 100 to 150 miles at an average speed of just under 25 MPH.
Then you should be fine training riding around a park 20 miles once a week. There is no possible way to improve your fitness and preparedness.