Mare needs some TLC to be sure. I start with the BEST quality grass hay you can get. Adding some alfalfa would not hurt. Adding a very good quality prepared grain feed starting with modest amounts and ramping up as fast as you dare. Adding oil is another way to add calories quickly. The reason you NEED to do this is because once lactation begins a mare’s nutrition needs really rise (will double over daily needs in many cases). If she’s wormy or has other gut issues you’ll have an inefficient conversion of food. That means more input to get the same output.
I’d also plan on having milk replaces and maybe colostrum replacer near at hand once the foal hits the ground.
Last, and far from least, a mare with sore feet won’t feed well. Prompt and correct foot care may even be more important than anything else right now.
You certainly have a “full plate” right now. Best of luck to you in this project.
G.