Beach Lake trails is a cross country running & ski trail system that opens to multi-use during the summer months. The area is ski-only during the winter when trails are groomed. Excellent trail system maps exist at the trailheads and at major intersections.
Since they're designed primarily as ski trails, the loops are cut to maximize hill climbing and descents. There are few hills of any significant length, but they're plenty steep to get you puffing. Some climbs are on the ragged edge between wheel spin and flipping right over. There are some handy cutoffs to skip the worst of the climbs if you so choose.
3,4,5 and 10k main loops exist, with extra warmup loops and connectors allowing ride variety.
Bring your wits with you, as bear and moose sightings are common. The area is also unusually mosquito heavy, so bring the bug dope in the event you choose to stop for any length of time. (15 seconds?)