Posted: Yesterday at 15:55
The Jolly Jogger - Welcome to the thread.
In my experience finding the right training plan for yourself is trial and error. I've never ran to a HR/Time based plan, but there are plenty of training plans that advocate exactly that. I go over three hours on some of my longer long slow runs, but there are plenty of articles about that suggest doing training runs longer than three hours actually have a negative impact. Personally I ignore those reports as I've ran all my best times based off five runs of 20 miles or more in training, all of which took me longer than 3 hours.
I've seen and heard of people completing marathons on all sorts of plans. I'm sure you'll be fine. The first one should always be about completion. It's a great feeling to cross that finish line for the first time.