My guinea pig would run away if I let her outside?

People say that they should go outside, but my piggies are really fast and I can't just let them down. They are too fast I could not even catch them. I take the top part of their cage outside and let them play in the grass, but I can't really afford a small animal pen right now. I really want to be able to take them out to run without keeping them in their cage! What should I do?