Velo, most of the evening training rides that I've done are 20-25 miles, with only one route offered (although there is often an unofficial shortcut). One of the nearby shop rides offer two ride lengths, but I think the slowest group does the shorter of the two routes early in the season and then progresses to the longer route--although I could be wrong about that.

Frankly, if I was in your position and trying to put a new ride together, I'd keep the logistics relatively simple and would likely just offer one route with 2-3 sub-groups to cater to different paces and abilities. I'd make it a length that was doable after work such that people could still get home at a reasonable hour, and I'd only offer a post-ride social option, say, once a month--again in an effort to keep logistics simple.

I'll share one other "like" of the rides I've done in the past. It's really helpful to have a bathroom or porta-potty that riders can use before/after the ride. One of the training rides I did met at school that had porta-potties near the school's ballfield. Another of the rides I did, did not offer similar facilities. Guess which one I preferred?