1) Be consistent, do the best you can to ride 3-4 times a week, and HAVE FUN.
2) Make one ride long, two rides moderate, and one ride short and fast.
3) Increase the length of the long ride but don't fiddle with the others too much right now.
Let's say your moderate rides are 25-30km, your long is 40+ km, and your short is 20 km. Once a week ride 20 km at a fast pace, twice a week ride 30 km at your normal pace, and once a week ride 40 km at your normal pace. Increase the long ride 10% or so from one week to the next. 40 km first week, 45 km second week, 50 km third week, .... Just use your own judgement if this is too long/short and adjust. Or, if the 40 km is hard, ride it as is for 4-6 weeks before starting to increase.
Once your long ride starts to exceed 50 km, you will have to pay more attention to hydration and nutrition. Figure about a liter of water per 15 km, and about 200 calories (kcals) per 15 km (from sports drink, cookies, fig bars, whatever.)
The first couple of weeks might be discouraging, but, if you keep riding regularly, you should start to see real improvement by 6-8 weeks.
Good Luck!
One simple way to get your saddle set is the way you mention, leg extended with heel on pedal. I find this is a little too high myself. But, it is probably correct within about 1/2 cm. If you experience knee discomfort, experiment a bit with the saddle height and placement of foot on pedal. Movement of just a couple of mm can make a profound improvement.