I can’t seem to get the fitness up on my two leases! One’s B the other’s a C. Not horrid, but I would like it to be A for both of them. I jogged them 1/2 a mile today, their energy didn’t go down at all. In fact, I think it rose. 
But anyway, my origional question: How do I raise fitness?
Personally I think that fitness itn’t a huge deal, it just helps them recover faster from stuff (but then again, i’m no expert!!). SO if they have good energy, i think it would be ok if you raced them.
I think you’ll be fine to race with your horse on A energy and B/C fitness. Racing will push fitness up quickly too (quicker then training I believe, so you may have peaked with training and you actually need to race your horses).
I don’t race when the horse has D or F for energy or F for fitness. I’ve had horses win races on C energy and D fitness.
Also remember that the ratings have a built-in margin of error, so what it says may not be what it really is. 
Everybody has different methods of training that work for them, for me though, Fitness basically has to be an A, if its not I don’t race because I get terrible performances. Others have an entirely different approach though, that works for them. You just need to play around with things and figure out what works best for you. 