First, you need to have a supply of hay to help your pastures. Secondly, realize how MUCH pasture can support your horses WHERE you live. In IL, it takes ~ 1 acre/horse/GOOD rain year. I keep 3 horses currently, I keep them on 4 acres of turnout/pasture/riding and schooling areas, and they are on pasture exclusively about end of April through end of November. I feed hay during the winter, and they still graze the stubble when they are turned out. I also keep hay in their shelter's manger, when necessary, and that sometimes means during a drought.
When I bought my place, it didn't look like it had 5 acres. The (previous) owners had my 3 acre, North pasture in corn. That pasture was exclusively planted by my horses and the grass hay I was feeding them. Horses are the best reseeders among all grass eating animals bc so many seeds pass through them. I believe most of your pasture will grow back.
I lost a lot of my south pasture through some mismanagement. I reseeded last fall, but they will be kept off of it most of this year to give the rye seed, especially, an opportunity to take hold. I don't let them on it during the fast growing Spring wet season, either, bc I don't want grass founder. My north pasture grows a lot of clover, too, and dandelions, and other edible weeds, so it isn't a factor.
Here's a link to my pictures from last fall's replant. Pasture Replant Project Photos by ducks4you_2009 | Photobucket
I triple tilled bits and pieces, and the biggest section was about 12 ft. South of the fencing. A few photos show that it took me from September to November to get the planting, done, and most of it got established before winter.