haven't been on a sunday ride recently but from what i hear they are now minimum 100km and can be quite fast and furious. sounds like the type of rides i would enjoy (and probably get dropped like a bad habit, guess i like getting dropped huh). unfortunately, tri training has taken me away from these rides, but alas, i will be back soon.
anyway, considering the distance these rides cover, as well as the pace, i'd say it's very good training for the interstate. so do join when you can. i know i will...