I do NATRC, and go around 40 miles in a weekend.
Where to start depends on your horse's current condition. If your horse is a total pasture puff (which I don't think Gidget is), you would have to start much slower. Assuming Gidget is in decent shape -- capable of an hour or so of walk-trot-canter arena work--, you can condition like I did my girl. (Who started off somewhere between pasture puff and walk-trot pony.)
First, walk. Walking is a very important first step. Walk four or five times a week, around five or six miles. Vary terrain, walk up hills, and add some trotting. When trotting, don't trot for a minute at a quick, forward pace. Collect into a slow-medium trot (around six miles per hour) and trot for five or ten minutes straight. Don't stop and give her a break. Go for longer and longer every day until she doesn't have a problem. (This will also teach her to pace her trot.)
At some cantering after she is good with trot/walks. Canter slowly at first; add it in with your trots. Cantering is actually easier then trotting at the same speed, so she won't have too big a problem stretch wise. Cantering is great to build wind and aid P and R recovery times.
Keep on building on this routine. Trot, trot, trot. That's the key. Vary your speed trotting sometimes. Also, get use to half seat, because posting for 40 miles is a no. Makes their backs sore.
That's pretty much how I conditioned my girl. She wins firsts and frequently gets super low P and Rs. Her's drops to resting in three minutes or so.
I also ride her four to five days a week. Remember that it isn't the miles you have on a horse -- you don't need 20 mile training days--, but what you're doing with those five-seven mile jogs. Also, a horse only has so many miles on it. Try not to waste them.
More isn't always better. Those completion weekends are stressful. They aren't something you want to frequently impose on a horse. You don't train for marathons by running practice marathons.