For doing long event rides, I found it very helpful to ride a Super Randonneur (SR) series with a randoneuring club. Preceding the SR series, many clubs also put on a Populaire of 100k, just to get the rhythm of randoing, riding with rando folks, doing controls, that sort of thing. The SR series is 200k, 300k, 400k, 600k with one long rest stop, usually at ~400K. That's good for getting used to pacing, hydration, fueling, and discomfort, that sort of thing.
However, I never, ever, put in long Z1 miles. My training plan was a competitive group ride in hilly terrain of 3-5 hours, once a week, then Z2 rides of 1-2 hours during the week. Once I'd done the SR series and knew what rando felt like, I only did that training regimen and the occasional 400k or double century. For a rider with a low FTP, I did OK, finishing in the top 10% or higher. It's all about precise nutrition and pacing. I use both a HRM and a PM. I put a hard cap on HR for long events, the number depending on length of event. For instance near the end of a hard-ridden hilly 200k, accelerate hard and see what your max attainable HR is. That'll be your cap for 300k, etc.
I found that going hard on hill after hill for no more than 5 hours, to exhaustion, while consuming one's max carbs/hour was the best endurance training for moderately long events like you're contemplating. Getting close to a target event, I did 2 or 3 mountain rides of over 100 miles or over 6000' or both, again to exhaustion. I found it helpful to find my limits and what that felt like. I didn't run, but I went to the gym twice a week until that wasn't an option due to energy and muscle limits. I did almost no formal intervals. At the most, 45" hill sprints. Riding hills of varying lengths, 50'-3000' is very good training.
__________________
Results matter