Hi Simon,
I'm a qualified personal trainer/keep fit instructor too and thought this could help me when we moved to Spain 4 years ago. But I never took the idea any further.
The reason why is that we live in a fairly remote village and most of our neighbours are not rich enough to consider paying for keep fit advice (plus teaching aerobics in the searing heat here is not something I would enjoy ).
From what I have seen in our area (rural Granada), there are two ways to do this. Either advertising on flyers, stickers, in bars, hotels, local mags, etc, and hope.
Or set up a gym. But of course that takes a fair bit of investment and could be a big risk (the gym in the town near us is under its second or third ownership in a year....).
However, if you are living in a more urban area you may well find more opportunities, especially if your Spanish is good.
Best of luck