Many Tokyoites like to come here for walks. It's a nice short easy walk, but nothing spectacular so don't expect some haven in nature. There might be a lot of people there on the weekend and the paths are narrow so you might get stuck behind some very slow walkers. Otherwise, it's quite pleasant. There's a cute temple there too!
Institute of Nature Study 国立科学博物館附属自然教育園 in Meguro 目黒 is a much better place for a walk, in my opinion. It's a magical slice of nature, probably not a must-see if you're a tourist but for those who live in the city, it's a beautiful and quiet place to get away from the crowds since they only let 300 people in at once. It's amazing such a place exists in the middle of Tokyo and so few seem to even know about it. Sure you have to pay 310 yen to get in and can still hear the faint sound of cars, but it's surprisingly peaceful with ponds, flowers, butterflies, and lots of different types of trees. Bring a sandwich and relax for an hour - lots of empty benches on a weekday! If you want to "go hiking" but don't feel like going away, this might be the perfect place to take a walk. There are some cherry blossom trees by one of the ponds.