What part of the lake? Sports LTD in south shore is alright for rentals, I believe they're a kona/specialized shop.
As for rides, kingsbury summit to spooner summit is the best bet for your time range. 7 miles from the trailhead to 'the bench' which is the highpoint and the halfway point. Now you can either continue downhill to spooner summit if you got a shuttle, or ride back into town and back up kingsbury grade (7 mile downhill then a 4 mile steep road climb back up kingsbury)
If you don't have a shuttle and don't want to road ride that much I'd just turn around at the bench and ride what you just did backwards to the kingsbury trailhead.