In my office park during my lunch break I tend to walk aimlessly around the parking lot and grassy open space just to get a little vitamin D (sunshine) in the middle of the day. There are many trees, large buildings, cars, bushes and other interesting obstacles that allow me to practice echolocation. This gives me an opportunity to walk places that I am not familiar with and practice different types of clicks, different volumes, different head postures, etc. I generally sight a target in the distance and then navigate my way to it.
I've had good success with the large buildings. I can see when they start and stop as I walk beside them, and I can judge their distance up to about 30-40 feet away. Further than that I can tell there might be a building there, but probably wouldn't be able to accurately judge the distance.
Trees are good targets, I generally attempt to make a tree my target from 40-50 yards away and locate it. Of course since trees are round, the response signal that will be reflected back to you will appear narrower than the actual tree. This is because a lot of the signal sound that strikes the tree near the sides, just glances off of it and does not properly return to your ears. The signal sound hitting the centerline of the tree will be reflected back to your ears nicely and it will taper off as it approaches the sides. For this reason I like to choose large trees as they will have the best response. Another good reason to choose a tree as a target is that since they are round, they look the same from all angles, as opposed to a car, or building. So it is possible to walk up to a tree and circle around it using echolocation. Circling an object is a good way to verify that you are well aware of the size, shape and location of an object without taking off your blindfold, opening your eyes, or using your cane (whichever your case may be).
Undoubtedly, you have some sort of outdoor environment that you can use to train echolocation. I would encourage you to simply explore at your own pace. Make mistakes, but be sure to stay just within your comfort zone, only straying for your comfort zone when you know you are in a safe environment and when you have calculated all of the potential misfortune that environment can present. Find targets, find routes, explore new sounds, new techniques, use some of the intermediate lessons and suggestions on this blog if you want to, but most importantly keep an open mind and OBSERVE! It's not imperative that you walk away from each session with something concrete that you have learned, but it is important to enjoy the learning process. The skill will come with time if you allow it to.