Learning to ride a horse for a city dweller?

I am very interested in horseback riding and have some experience. I live on the outskirts of Portland with many stables nearby and several offer lessons for $20-60 an hour. My question is this: Once I have learned basic to intermediate skills will many stables let you pay a fee to ride one of theire horses for an hour or two without lessons or do you have to board your own horse to do that? I want to learn riding but don't want to take the plunge into full-fledged horse ownership.