While I agree that you can't make an 18 yr old horse look like a young filly again, I believe that you can bring her back into good enough shape to look like a younger horse in good shape.
That's just my opinion, and assumming the mare isn't suffering from a physical ailment that cannot be corrected, ie; cancer ect. If I had the challenge of bringing this horse back here is what I would do.
1. Get her on a good worming schedule, rotating wormers (which you have already done).
2. Get her to a equine Vet, not just a large animal Vet, some one who specializes in equine medicine. Not a knock on large animal vets, but a vet specialized in an area is always better than a general Vet, haul her if you need to. Have the Vet do an exam, draw blood for a complete blood test, this well check a lot of the organs and their functioning, also give the Vet a direction on things such as cancer, kidney failure etc. Also have the Vet anilyze a fecal sample and check the heart (they should check the heart anyway in the exam, but explain that if she checks out okay you'll start working her and want to make sure the heart is up to it).
3. Assuming she checks out okay, start working her slowly, as stated a few times already. I would work her about 5 minutes a day (unless she is struggling then stop sooner), this is after you have warmed her up, make sure you cool her out after, walk her until she stops breathing hard. Do this 5 times a week giving her 2 days off. Increase her work out time every week.
4. Control her diet, at first cut the hay back a little and start increasing the grain slowly every week, as you are with her work out times. Don't let her free graze at first, your trying to work off some of the belly. You can give her some Alfalfa and Vitimins. You can also add corn or soy oil. When you have her working a solid 15 or 20 minutes a day then you can put her back to free grazing and/or more hay. Max the grain at about 3/4 of a coffee can twice a day, again, bring her to that level slowly. The alfafa will help with a hay belly also, but it may make her too hot, it effects horses differently, some can handle it and stay calm others go nuts. Make sure she has minerals.
There are different ways to make her work harder in a shorter amount of time, riding will work her harder that work in a round pen or longe line. There is also ways to increase different areas of her muscle mass, ie; hip or back. Working in a trot will build muscle quicker than working in lope. But that's a long way down the road, right now if she checks out with the Vet, she needs work, just excercise to get her in shape, don't worry about doing it in a trot or lope, just do it.
I have a 11 year old brood mare who is on her 7th foal now, she is in great shape and a great ride between foals, with luck she will have 7 more.
Sounds like a fun and difficult challange for you, I you decide to do it good luck, if you decide to leave her alone I'm sure she has a great home and will be just as happy not working out.
JMO
Alan
