I find these conversations interesting because everyone is so different as to what works for them physically and mentally.

You and I are very similar in that I can easily spend twice or even three times as much time on a bike as I am willing to tolerate in a gym. I love biking so much that I've built up an absurdly high level of endurance, more than I ever could do on a stationary machine. So obviously, we are at very different stages in our development as riders, but I think we're similar in what motivates us.

I don't know if any of this applies to your situation, but the way I ramped up my riding was by taking advantage of what I see as the main appeal of cycling, unlike other forms of exercise, this one happens to be a pretty good form of transportation. So, what I would do is pick a destination I wanted to go to anyway (usually lunch in my case, but there's been zoos, sightseeing, stores, etc.), plan on cycling there, stop at the destination for an hour or two, then return on the bike. I started with destinations that were about 10 miles away, and gradually, they just got further and further away. Last Saturday, I did one that was 65 miles away (130 round trip) and it was only about my 5th longest trip this year. The destination has two advantages for me--motivating me to want to get there, and the extended break lets me regroup for a long ride home. For me, at least, two rides of 50 miles in a day is a lot easier than one ride of 100 miles, and that scales up and down.

Here's the thing for me--because I get the kid-like feeling like I'm going somewhere fun, it doesn't feel at all like training and exercise. Sure I can feel it physically, but I don't mind it as much as I would just churning something stationary. I don't do all that HR monitoring and timing my cadence stuff because that too would make me feel like this is a task rather than an outing. I do monitor my speed and distance, but that's just because I love the geography aspect of things and I also like to see how fast I can go (it's pretty damn fast, BTW).

I just realized that "if I were you" is one of the silliest expressions in the English language. None of us can tell what might work for you, but I think it's important to see the big range of ways people approach this--I've gotten really good at riding by ignoring a lot of people telling me it must be done one particular way.