It all depends on your aims and goals- I am guessing you are saying you want to be able to ride longer to explore the countryside a bit more.
If you try to ride hard all the time (for example every commute) you will only wear yourself out. The real gains come from recovery (of course you need the overload first)... and most people will ride too hard when they should be taking it easy and ride too easy on the few rides a week that need to be properly hard.
Patience in building up the time riding is important too as is consistency. No point in getting in a good couple of weeks riding and then do nothing for the next couple of week or knackering yourself for a month only to pick up niggles.
Strictly speaking the way to "get fitter" is to ride as many easy miles as you can fit in. Cycling is a very aerobic sport and the very hard mid week efforts should be a final tune up for speed if and when needed in the year. Of course this is not the most fun approach in the world and most people do not have time to fit in a decent volume of riding hence the low time/ high intensity approach many here suggest.
I would say to start with at least, gradually increase the easy week rides and extend the length of an easy weekend ride. Going nuts on hills repeatedly or half killing yourself on a turbo might be fun but can probably wait a few weeks
Also, although I might have said ride easy "miles" judge duration by time as distance has far too many variables to judge volume.
That would be my thoughts anyway but I am sure many would disagree