how much do you worry about a mare that is bred that is older? Light riding, no riding? Etc? 

At our place, once one becomes a broodmare they are no longer ridden period. Not saying riding is bad for one, just stating what we do.

This mare is so broke, but also has great papers. She has no issues keeping her from running. She loves to work, but don't wanna loose the baby. 

So how do the wild mustang's out west ever have babies if it's so easy for them to abort. 

They don't get pampered. 

Wild mustangs aren't out running barrels or competing . Big difference. I would never ride a bred mare. One spends so much getting mare bred. When I was breeding, I had bred barrel mares who won alot but they were retired from barrel racing to be mom's. Running a bred mare can be an accident waiting to happen. Suppose mare slips & falls turning a barrel. Hauling here & there, alot of virus's going around. I don't know any good breeders who run their bred mares. If you want to run a mare, don't breed her. JMHO

How old is she?