The Waldo Lake Loop is actually called the Jim Weaver Loop Trail #3590 and this is a wonderful trail that goes around the entirety of Waldo Lake. Most people make this loop into a one night backpacking trip as there are a few places where you can easily backcountry camp around the lake.
You can start the loop from Shadow Bay Day Use Area or up at the day use area near North Waldo Lake Campground. The write up described here starts from Shadow Bay because the 5 miles from Shadow Bay to North Waldo Lake Campground are entirely through the woods, which is better done at the beginning of your hike, rather than at the end when you are tired. After you’ve hiked 15 miles it is nice to have some views to look at while hiking the last 5 grueling miles of a 20 mile trek, especially if you choose to do this loop as a day hike.