As soon as I have my own baby (any day now) and I have a couple of weeks to recover, I plan to get started. Probably start easing into getting back to work with the horses around Jan 1 and we will probably go to our first show in April. So to answer your question, 90 days prior.
We started really too late last year and were not ready at all for our May shows and I don't want that to happen again. We played "catch up" all summer and worked really hard late in the season leading into Nationals and I really felt like the horses needed some time off (well, me too) October - December. As far as feeding goes, the only major change is they get lots of good hay to munch on....almost free choice....but they are outside most of the time and it has gotten really cold.
IMO The most important right now for you to do beyond working her to get ready for showing is stick with a really high quality well balanced diet, keeping up on the supplements, worming, teeth, farrier, grooming, etc. and you can work on stuff together that keeps the mind working without working up a sweat. do some halter work, work on obstacles together, do some trail work - or basically just going for walks, working on walk, trot, whoa, sidepasses, etc. Just to spend time together and do a little but of easy work at the same time. Susanne put it well when she said "grade school P.E. -- organized playtime together --
By the way I noticed I'd promised all my boys a "vacation" when they got home from nationals and they spent a lot of time in the big pasture together just being horses, grazing, rolling in the mud, running around and kind of having a "I want to be left alone" attitude. to the point that when we had friends over and wanted to show them our minis, the horses would just ignore the guests or look at them suspiciously and over the last couple of weeks I can tell that has changed and they seem like they want lots of attention again -- like they are ready to get back to work now. Hopefully they will all be eager and ready in January.