SAFD:
I’ve been to every Marathon Weekend since 2012. We have attended in groups of 3 to 13 and have done no parks, all the parks, and everything in between. I’ve also had periods with an AP and, much more often, without one.
As an out-of-state, mostly one to two trip a year person it would be odd to go to Disney and not do any parks. The only year we didn’t do any was 2013, which was my (and my sister’s, the only other constant on all the trips) first full Marathon. We were still very much noobs and thought it would be weird in the parks without kids (an opinion we’ve left in the past). It was a short trip that was almost entirely focused on finishing the Marathon. It was a great trip even without parks but I don’t think we’d ever do things that way again.
Most of the years since we’ve been running Dopey and burning the candle at both ends. This is definitely a passable option, but it wouldn’t be my current choice (both for race considerations and Disney considerations).
Race-wise, the sleep considerations for Dopey lead me to prefer to do parks only up through Thursday. I like having the later days leading to the longer races clear for recovery and sleep.
Disney-wise, I have been growing increasingly disenchanted with the day guest experience at the parks. We stay off-site most of the time during Marathon Weekend, due to the size of our group, and the disadvantages for off-site visitors (they really fit this classification much better than perks for Disney Hotel Guests) make visiting the parks on such busy days completely unsatisfying. I much prefer, and am fortunate to have the option of, doing fewer parks during Marathon Weekend and scheduling another trip on-site during a lower crowd period (I know these are growing fewer and farther between) later in the year. Even more than that, I prefer to spend my time at the parks at holiday parties or after hour events. Events where
Genie+ and the entire idea of multiple lines doesn’t exist.
TL;DR- Any amount of park time during an RD trip is feasible and can be enjoyable. It’s really individual preference both factoring your enjoyment of the parks and your race planning (and whether your race planning can supersede your own and your group’s vacation plans).