Quoting Lucy Rees again:
'We have no idea what amount of space horses consider enough. Though feral horse bands stick together, they wander kilometres every day. The pottokas have over 3,000 acres to live on, but they find it small. If anyone leaves a gate open they go, up to 15 km easily. Sometimes they come back. They just like going walkabout.'
We can't give our horses unlimited space, but we can take them 'walkabout'.
Of course, it depends how you ride out - a frantic, unbalanced charge across the fields while you hang desperately onto your horse's mouth isn't going to do anyone any good, but a relaxed ride out (at any speed) can be good for your horse on many levels, so don't apologise for 'just going out for a hack'! And walking out in hand can be, I think, equally good for the horse, and in some cases better!
An arena is a great place for lots of types of work with a horse, but it's a real shame that going out for a ride is often so underrated, whilst a 'schooling' session in an arena where both horse and rider are getting confused/frustrated/bored is somehow judged to be 'proper work' - know which I'd recommend.