A walk around the lake (13 or so miles, to wit) is a great way to start the weekend, but be on the lookout for runners and bikers, both can be a bit space competitive. The tree lined wide path around the lake is enjoyable and provides for both nature and people watching. Unfortunately, there is just one spot along the route with portable toilettes and these porta potties are in sad shape. Keep this in mind when planning a long walk.

White Rock Lake Park is in Dallas. It is a great place to unwind and relax.
The trails are nice for walking or biking. You do not need to bring your bike, because there are always rental bikes around the lake. It is very flat though, and you will not find hills or steep areas.
I love going when it is not too hot. There is always a little bit of a breeze around the lake.

I lived just off Lanshire Rd, which is the northern boundary of White Rock Lake Park, for the past 5 years. My wife and I would walk through the park at least once a week on average and would have a picnic lunch down near the bathhouse theater at least once a month. There are lots of cyclists and runners on the trails in the morning and evening hours before and after work during the week and lost of family gatherings on the weekends.A great place to get away from the hustle and bustle of city life for a few hours. the Dallas Arboretum occupies a beautiful space on the south side of the park and Flagpole Hill with picnic and playground facilities is on the north side. The Bath House Cultural Center and theater is on the east side and the dog park and trails are on the west side. A cycling and walking trail runs almost the entire length of the lake from the dam on the southwest side to Flagpole Hill on the north side to the Arboretum on the southeast side.