Just looked at my mileage from the last time I rode it and it was 63.8mi parking at Arbolado, clockwise, and climbing to the summit before the end.

I think it'd be really optimistic to do that ride in 3.5h, even skipping the summit. More reasonable might be to summit from Arbolado, go down the south side to the Draeger's Market at Blackhawk Plaza, get a beer/bite to eat, then ride back up the south side to the Junction, and to the summit again if you want the mileage.