Spinning is hard... its no joke. Just because lots of girls do it some lads think its handy, and it definately isnt.
Even the lads who I trained (on bikes) with would be wrecked when they put fixers on (ie removed the rear sprocket ratchet meaning you have to spin your legs all the time).
The trick is to get the efficiency of the movement. To not have your legs fighting against one and other. This does not nessesarily mean you are or aren't fit, it just means your legs know what they are doing. You will probably notice this because, at the start your legs will bulk slightly, and then as they get their act together they will lean down to smaller than they were when you started.
Once you get it, then you can start being very clinical and careful with your training. You can spin at
just under your aerobic threashold. And, then bring your pulse up into anaerobic levels for a few minutes, and measure recovery rates. When you are being clinical like this, you can train really efficiently.
Its good. Keep it up. I am sure there are lots of pretty legs and bums to look at too while you are at it....