Hi B.
Well let me start by saying that I am no pro on Shadow Boxing. It takes great will power and determination for me to do it. I just don't like hitting the air...
But, I should also say that I am convinced that it is a VERY valuable tool and when I concentrate on it and do it, I notice allot of improvement in speed, conditioning footwork and BALANCE!
So my personal advice is this, pick a drill from focus gloves and do it in the air. Think of a footwork combo and do it. relax and be in balance! Don't worry about speed.
Also, don't worry at first about doing something different every two seconds.
For example, lets say that before you begin, you think, OK, I'll jab-catch then cover-tie-up, three knees, push then two round kicks. Important to SEE in your mind's eye, the opponent, try to "feel" the contact. A Japanese Ninpo teacher of mine said, when you can punch the air, and it feels like your hitting a bag, you understand!
So again, balance relax and SEE the bad guy. Then next round, change on piece. Imagine that after catch and cover, you move in for clinch and he shoot on you, push away with the L foot work.
Also do it on the ground. I think starting with pre-arranged combo's really helps, learn to really see and feel the bad guy, BALANCE and relax, forget speed and power, after some time, you will not need the training wheels and you can free flow.
When you see people going fast, it shows that they have good visualization skills, speed comes with relaxation and BALANCE..
That's my opinion and advice, I too would like to hear others opinions as I said, shadowboxing is not my strong suit ( I dislike any aerobics that don't involve contact)
I'm working on that
Best,
-Rick