One of my favorite afternoons ever was spent on someone else's bike. I simply walked into Bicycle Sport Shop, took a look at the various bikes available for rent and picked one. Before I knew it, I was cruising along the Town Lake Hike and Bike Trail like a professional.
It can be overwhelming to get an initiation into cycling in Austin if you don't have the proper equipment. Fortunately, plenty of bike shops offer rentals that are both affordable and easy to ride. Here's a round-up I wrote about the best place to rent a bike in town. Most places require an ID and a payment of around $30 or $40. The easiest thing to do is to go to a rental shop that's near the trail you want to ride -- that way, you don't need to deal with attaching the bike to your car.
Go out and give it a spin. Happy riding!