I mostly agree with RiosDad. I think, as long as you exercise moderation (like no ten minute gallops or something), they should be good to go. They get 24/7 turnout, right?
If you're still concerned, what I do with Lacey when I'm building up her fitness is basically human interval training, on the lunge. I warm her up really well at the walk (both directions, I like to do lots of direction changes to keep things interesting), then I have her trot for about ten/fifteen minutes with downward transitions about once every couple minutes, then straight back to trot. Then I have her canter until she's breathing hard. Then we switch directions, walk until she's not breathing hard, and repeat. Then, I ride at all gaits (but mostly walking) for about an hour. It works pretty well for Lacey, she's pretty dang fit right now and we've only been intensively doing that for the last month or so. Of course, I was working her all winter but she was working hard. :)
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.