I've had 2 horses that choked. One was starved when he came here and bolted his food, rocks and letting him get filled up (that took a while) solved his choking issue. The 2nd one was a chronic food bolter that just could not seem to slow down, even with rocks in her feed pan.
So my questions are, how old is the mare, do you feed her by herself or in the herd and what is her place in the herd? Has she ever been starved that you know of?
Soaking the feed will help, it will be more slippery and less likely to get stuck. I also added vegetable oil, that seemed to help a bit too.
Be very careful for the next 2 weeks, she could very well choke again if she caused any lesions in her esophagus from this one. I would really wet her food down for the next 14 days. The vets at OSU recommended I switch from the over the fence type feeder bucket to a flat pan and had me put a couple of bricks and some fairly large, say fist sized, rocks in there to force the horse to pick around the rocks to get his feed. I fed solo, in a stall so no one could pressure the horse and gave him all the time he wanted to finish his bucket.
Sometimes if you can stick a hose in their mouth and get them to swallow the water it will loosen the choke a bit while you wait for the vet. I also have massaged the mass to try and break things up and get them moving. I think choke is worse than colic to deal with and it can recur a lot more often. I NEVER wait any time at all for a vet call when I see a choking horse, if it completely obstructs things, you can have a dead horse very quickly.