By ring Work I mean only in an arena or arena type setting. That means there is nothing challanging for your horse to do so she really doesn't have to use all her muscles. Think of a biker who only cycles around a track or on the street verses one who climbs mountains.
When I start a horse back to work (one that has had a real long time off), I start them for about an hour in my fields which has some hills to it. I'll do a lot of walking for the first week (4 days) and introduce trotting by the end of the week. On the second week I'll take them out on a trail for an hour or 2 at the most. On the trail I try to follow my method of doing a Hunter's Pace - walk down hills, trot the flats, canter up hill. After about a month, I feel my horse is ready for a typical 2 to 4 hour ride but I still take it easy for a while and monitor her. BTW, my horses are on 24/7 turnout.
It sounds to me like you are not pushing your horse enough.