If you have ever walked around the Lafayette Reservoir or The Res as it is often called, you might remember that the Lakeside Trail is 2.7 miles around, mildly hilly and makes a nice walk for almost anyone including dogs on a leash.
The nice things about the Res are, you can park right next to the trail (expect to pay either at the meters or in the lot), you can walk at your own pace on a paved road with lots of room for everyone, and best of all, you are all done in 30-45 minutes if you can walk at a normal pace.
The not so good things about the Res are that everybody knows about it and it can get crowded, it is paved and thus is harder on the joints and it gets boring after you walk it a few times.
The next step is to try the Rim Trail, a fire trail above the Res that goes all the way around but is longer, close to a 5 mile hike. The good things about this hike are that you will get a lot more exercise, the trail is unpaved and therefore easier on the body, there are fewer people and more nature and you can park for free on the street at the end of Campolindo Drive where there is a public access to the trail. This trail is very hilly, lots of steep uphills and plenty of anaerobic hiking. It is definitely not for the unfit. If you are starting out on a mission to get in shape, this is not the trail for you. Go for The Res for a couple months first and then try the Rim. Another option is to walk 1/3 of the way around and then turn around and head back. That will cut the distance and one of the most steep hills.
You will quickly notice that avid hikers and walkers frequent this trail and they will speed up the hills, blazing past you with strollers and toddlers in backpacks. This trail takes some getting used to and you will soon get in the groove and be able to keep up with all the 80-somethings that used to pass you with ease. Give it a try! There is an excellent description of this hike and how to get to The Res on the Bay Area Hikers website http://www.bahiker.com/eastbayhikes/lafres.html