I just bought a mare who is about 200-300 pounds overweight. I'm going to ration her feed, but I'm worried about riding her. What would be a good riding schedule to help with the weight loss without overworking her?

Just start out slow and build from there.

Agree with above poster...start out slow as if comes g back from injury. Walk 5-10 mins first week or 2 then add a little trotting a few weeks then lope. I probably bring mine back too slow but the vet has always told me 5 min increments 

Here is a funny. Several of us went to Ruston LA to a huge barrel race. A girl was riding this obese horse. That sucker had dimples everywhere. He was by far the fattest horse there. I was thinking, I hope she doesn't kill him. Well, fatty got out there and placed in the 1D.  This was really tough competition.  About two years later fat boy was back again and I was riding around with the girl for a few minutes.  The girl told me she was thinking about buying him.  I told her he had shocked us all before and just think what he could do if he was in shape!  I do think you need to have a blood test run on the horse to see if it has IR or a thyroid problem.  

Promoting more movement in pasture is always a good start, and you can do so in very small ways or very large ways depending on your situation, finances, etc.

I don't have a fitness program suggestion for you but a feeding suggestion. I have an easy keeper, you have to starve her and ride her hard to get her fit. When you cut her feed back she starts doing obnoxious things in the corral.   I started using the Hay Chix heavy duty (cuz she is brute on everything when she doesn't get fed!) slow feeder.  It has changed our lives for the better!  She has something to do for most of the day.  I fill that thing with grassy hay and it takes her until noon or after to get her morning feed eaten.  No more rubbing her teeth on the fence or rubbing on the panels so hard she bows them out.  She is looking better this year as well without me having to long trot for hours.  Just a thought for you!  :)