Last year I ran two ultras and four marathons, all quite slowly, all but one with negative splits, and it feels very good. I completed two marathons on two consecutive weekends, and enjoyed them both.
Before that, I was trying to improve my speed, running 1-2 races per year, and that definitely was less healthy for me. I did not like tapering - last week before the race just felt terrible. I hated the recovery - I'd rather work out than rest because of sore muscles.
So, for me trying to run one marathon per year with full effort was not healthy and not enjoyable at all. Running multiple ones with enjoyable speed, not hitting the wall ever, with negative splits, and no pain is a lot of fun. I am just staying in shape all the time, and enjoying a good scenic race whenever I feel like it.
We are all different. What works for me might not work for others. I am not cut out for fast running: I do not look like a lean gazelle. On the other hand, having considerable muscles and thick and strong tendons allows me to run injury-free, even though it prevents me from running fast.
I have lean friends who have real chances to qualify for Boston, (1-2 minutes short), but keep spending considerable time every year limping. They are much faster but more prone to injuries.