Thinking back to when I started out, my big improvement came when I did the following:

1. 1 3hr, preferably 4hr, ride on the weekend.

2. At least 2, preferably 3, 1hr rides during the week.

If you can do a second ride on the weekend of any length that's a bonus. If you can do the weekday rides with some intensity that will also help a lot (e.g., doing two or three 10-15 minute segments above your comfort zone).

If I had to choose, I'd say a 4hr weekend ride and 2 weekday rides is more valuable than a 3hr weekend ride and 3 weekday rides.