The waymarked trails at Croft Country Park (map) offer both a family/beginner off-road trail (Blue), and a more technical trail (Red) with features for the more experienced cyclist or those wishing to improve their skills. Ride them several times for a more intense workout.
Open all year round and free to use, the trails are easily accessible. They are within a mile of Swindon centre and train station plus there's free car parking at the start. There are even a few picnic tables should you wish to stay for a while.
Whilst built on land provided by Swindon Borough Council, the trail is managed by the MB Swindon club and has been built entirely by volunteers and is entirely funded by donations. So if you do come down to enjoy the trail, or have already enjoyed it, then please consider the "Support the Croft Trail" sidebar, join the club or come down on one of the trail building days.