Let 'em Have At It. What wonderful opportunity for your horses!
This is as close to being "natural" as they will ever get
Growing up, my horses ran on 100 acres with the Beefers. My current horses are only on 22 acres of pretty steep hills, so they stay in good condition, roaming the entire parcel.
I did cross-fence and have five pastures but the gates are always open. What I would suggest is to ask your landlord if you can fence off a huge paddock area (50' x 100') and a 1 - 2 acre emergency pasture, attached to the paddock. The paddock should be attached to the shed, if possible.
That allows you to bring a horse up close to the barn if it's sick or injured, plus you can close the horses out of the main pasture if you end up with an ice storm.
Last year we ended up with ice so thick (in southern Middle Tennessee) the horses couldn't break thru it. There was no way this side of Hades and Back I was letting them out in the big pasture on that stuff.
They ended up being in the "hospital pasture" for four days, until the ice finally melted to where they could break thru it.
My only warning is if you have easy keepers, be prepared to buy grazing muzzles and make them wear those muzzles from sun-up until sun down.
You don't want to be dealing with metabolic issues.
Our land was an old cow farm - like your pasture, nothing was ever done to it, except keep it bushhogged. It is really high in iron, therefore low in copper and zinc, two of my TWH's ended up with metabolic issues and I have only taken their muzzles off them in the last couple weeks.
Ride them around the fence line before cutting them loose, but let them just run, run, run and be horses. Call them up once or twice a day and give them something, even if it's just a couple hay cubes for treats.
With 40 acres you need to keep them coming in on a daily basis so they don't get to the point where they don't want to be caught.
If you have a 4-wheeler, dirt bike, or just use the pick up, make it a point to drive out to them, often, and give them treats in the field. That also keeps them easy to catch.
Believe me, going to them and handling them at liberty out in the field, will get you a whole bunch more than always trying to catch them to bring them to the barn for work. That way when you do want someone for work, they will allow themselves to be caught.
Teach them all to lead from whatever vehicle you go get them with. Mine will lead from the pick up window, off either of the big tractors, the lawn tractor, the 4-wheeler. If I'm on the John Deere, I can lead two at a time because I can steer that tractor with my knees
I am really excited for you! BIG CONGRATULATIONS! I hope this all works - any chance of being able to buy the land at some point?