I recommend three times a week at twenty miles. I am no longer going long distance. I average about twenty a ride now .I was forced off my road bike two years ago by wrist issues and multiple surgeries on it.
I ride an upright touring bike with high bars.It weighs more than forty pounds with rack and a couple of bags on it. I ride into the wind starting out. Also ride uphill a lot ,as I live in Utah.I do thirty to thirty five several times a month. This keeps me strong and fit and I know my body and I could easily train up to longer distance if I want. I think you need to to make the time to do twenty miles three times a week.
I am 68 , retired and have no time issues.I enjoy the ride more than ever.