I say this to everyone and I'll say it again.
Don't underestimate the power of just walking. Walking, walking and more walking. Actively walking forwards, over varying terrain, hard surfaces, soft surfaces, uneven surfaces. Nothing is better than lots of walking to get a horse back into shape without compromising joint and soft tissue soundness in the limbs.
If she has had 6 years off, I would only walk her for a minimum of one month, 5 to 6 times/week. Yes, it gets boring, but being bored for a month is much better than having your horse break down in a month due to too much work, too soon.
Once you've done your month of walking, her limbs should be conditioned enough and her cardiac fitness good enough to start introduce short bursts of trot work and later, canter work. Gradually building up the periods of time that you spend in these gaits until she is back at an appropriate level of fitness.
Remember, if you had not gotten off the couch for 6 years, then suddenly had to go out and carry a 15kg backpack and spring up a hill - you wouldn't pull up feeling too well and would be sore for days, or weeks and probably give yourself sprains, strains and muscle tears. You would be put off doing the same thing again and start to dread going for that run.
Same thing with a horse, if you get on and push them to the point of soreness and exhaustion straight off the bat, they're going to resent getting worked and associate it with soreness.
Just walk :)