I know some folks who are the type that will not ride for weeks, and then decide to ride the horses for 4 hour trail rides. They have one mare that has a six month old filly. Sometimes (not often) they will pony the filly out on the streets - luckily the roads are fairly quiet. They decided yesterday to go on a four hour trail ride (horses haven't been ridden hardly at all for the past few months). They ponied the 6 mo. old filly along with them - is that a bit excessive for a young one?
On one of my family's horseback trips, one of the packhorses had a 3 month filly with her. We covered at minimum 10 miles a day over rough terrain (no trails, brush above our heads, steep hills, creeks, etc.) The filly was loose and she kept up just fine. If wild horses can do it, I don't see why it's asking too much for a foal to do it, provided they were doing most of it at a walk.
Not ideal, I don't think, but likely far from the worst thing they could be doing with her. How are the horses kept when not ridden? If stalled or in small runs or paddocks, I'd say it's more of a problem than if they are kept on large acreage and the filly self-exercises.
If I had a baby out on 30 acres with my horses, I would think she could do four easy hours without any problem. We actually ponied my friend's two yearlings on a three hour ride Saturday, and it never occured to me that it was any sort of problem. We mostly walked, with maybe a total of 20 minutes of trotting, and we stopped pretty often (mostly to adjust their saddle pads they were carrying). They seemed to handle it all fine. The way they gallop and play, I don't think it was an unusual amount of exercise.