Well, I called the shop and asked for more info. They said the speed is about 14 mph and they "wait up for everyone", and that I should be fine as long as I do these kinds of distances "regularly". I wouldn't say "regularly", but I did do 16 or 17 miles this past Sunday and was fine, and have had days where I've done about 12 miles total while running errands, and I think my usual average speed is about 12 mph but it's hard to say because there are a lot of stoplights/intersections holding me up sometimes and that throws off the average. So it sounds do-able (I hope--my concern is with the speed) and if the weather looks good this Monday night I may show up. If so, I'll be sure to post a ride report!