I had a ratty old apple tree that I cut down years ago and then let it grow back from the root. After about 10 years its producing good apples again and now I want more. I don't know what kind of apples they are but they're very hard and very sweet
/johnny (not appleseed)
Apples don’t reproduce true from seed.
What you can do is find some rootstock and/or someone who is familiar with grafting and graft some sprigs of your tree on to some appropriate rootstock.
What you may have is the rootstock itself. Since you mentioned you let it grow from the root.
IF so, then just root some cuttings next spring when it starts to bud out. Lots of places on the net that give advice/tips on that. My only added tip would be to pick a root/transplant day from the farmers almanac calendar.
Go for it. Get several started so that if something gets one maybe the other survives. Nothing to lose by trying.
Homemade Apple Pectin
3 lbs sliced unpeeled tart immature apples
4 C water
2 T lemon juice
Boil until reduced to half. Strain through cloth. Boil juice another 20 minutes. Store in fridge, freezer or water bath.
YMMV/trial/guesstimate on how much homemade pectin to use in canning.
KEEP THE TREE!! Grow some new seedling from seeds. When you have enough limbs that are close in diameter on the new growth on the original tree, graft the limbs from the old tree onto the new seedlings. BTW, you can graft apples onto pear trees.
Don’t waste time on seed, since you want to reproduce that fruit; they won’t come true.
Either take hardwood cuttings, and root them; or buy some rootstock; check with your Extension Service for which one(s) are recommended for your area, and the size tree(s) you want, and graft onto them.
If you don’t know how to graft, a Master Gardener in your area can either show you, or point you to someone who can...and may even do it for you.
If it were me, I’d go ahead and plant those seeds. The trees that grow from them will still be apple trees, although they might be different from the mother tree. If you decide you don’t like them, you can graft branches from the old tree onto the new trees.