A radar gun should be placed as close as possible to in-line with the path of the pitch. However, being a few feet off in one direction or another usually won't make much difference.
You can expect to be throwing slower than younger players at first. It will take a while to develop arm strength and technique. That's a good thing because if you were able to throw 80+ mph the first time out you'd destroy your arm almost immediately. It takes time to build up the muscles, tendons, ligaments and bones to handle the stresses of pitching at high speeds.
A mound usually contributes 1 or 2 mph to velocity but that isn't always the case. You have to learn how to transfer the additional momentum you get from throwing off a mound to added velocity on the ball.