For backpackers only - this may seem like a short distance from the trailhead, but it is a steep incline the entire way up. With packs, it took us close to 5 hours with short breaks. The top is at around 10,500 feet. You can't camp directly by the lake, you have to go to the right of the top of the trail. Once by the lake, it's a little hard to see the continued trailhead up to the top of Mt. Herard, but just look for the cairns (trail markers/stacked rocks) that will guide you. Again, the top to Mt. Herard doesn't sound like a long distance, but there is bouldering and scrambling involved and will take longer than you think. I do not recommend taking dogs to the top or children.
We saw all kinds of wildlife, including 2 bears. Be sure to hang your food a good distance away at night or when you aren't around! Woke up to deer, saw plenty of little marmots, the local beaver, and lots of trout in the lake! So beautiful. On the other side of Mt. Herard, we also spotted a few Elk relaxing. Temperatures weren't bad at the beginning of July. With the proper clothing and gear, we didn't get too cold. Watch the movement of the clouds or storms coming in if you plan to hike up Mt. Herard. It could get very dangerous if you're caught up there in a storm.
Do this trip! It's breathtaking and quiet - there weren't many people up there at all for our 4 days. It was very peaceful and again, rewarding. Please note that you must have a 4WD vehicle to access the trailhead either from the North or through Medano Pass from the Great Sand Dunes. You will not make it without 4WD.