More thoughts for the OP:
When I started riding again in, hmm, must have been ~'95, I had exactly the same aspirations as you. I did it: first summer getting some fitness, second summer a couple centuries, 3rd summer the STP one-day. (Stupid to do the 2-day IMO. Who wants to put on a pair of cycling shorts the day after a century?)

So, how I did it:
1st summer I started by trying to ride up a nearby hill and back home, maybe 20 miles and 500' of climbing, some of it kinda steep. I thought my lungs would come out my throat. They didn't. After I could do that ride successfully, I started doing one longer ride/week. I'd ride away from home until I was quite tired, then ride back, mostly staying on flatter roads. Endurance doesn't start to build until you start enduring. It has to take some mental discipline or nothing much happens.
2nd summer, I bought a better used bike. I built on the first summer by riding hillier roads and working up to riding away from home for 50 miles. I did at least a couple solo centuries that summer.
3rd summer, I started doing long group rides, up to 125 miles, and then rode that one-day STP with some folks I met on the group rides.
4th summer, I bought a modern road bike and started doing more difficult rides. STP isn't difficult, it's just long.

And oh yeah, intensity definitely should not be avoided. To the contrary, it should be sought after. You can tell you're riding hard enough when your lungs start to hurt after an hour or so. Of course you can't keep that up forever. I've been sick and haven't been out on our tandem since July 1. So today we took the tandem out on a group ride with some of our buds. The first hour was mostly uphill. We did over/unders that whole first hour. IOW, we were within 3 beats of our lactate threshold HR, sometimes above, sometimes under, for the whole hour. Then we went off the back and caught them at the lunch stop. After that we did mostly touring pace, but at the end my average moving HR was 124, which is 90% of my lactate threshold HR. Yeah, my average for a 2-1/4 hour (moving) ride. That's how you get strong. Tomorrow we'll do 45' of zone 2 on our trainers and then lift weights at the gym for about 75 minutes. Etc. Our target for the next few weeks will be 7-8 hours/week with about 2-1/2 hours of that in the gym trying to get some muscular endurance and strength back. By next July, I should be able to do 10,000' climbing rides again.