As Micek says, they are faster than you think. We looked after one for a fortnight some years ago, while our friends went away on holiday. It was always escaping and we were constantly combing the garden for the little blighter. I was glad when that fortnight was over.
I took my elderly neighbour's dog for its annual vaccination yesterday and there was a lady waiting in the surgery with a tortoise on her lap. I'd never seen a tortoise in a vet's waiting room before and curiosity got the better of me. She explained that the poor thing was confused. The sudden cool and wet weather had sent him back in to hibernation and he wouldn't feed.
I had trouble keeping the dog under control, he thought it was a crusty pie!
When I was very young and we had just moved into our first house, we realised that the lady next door had a tortoise in her garden that was fastened via a hole drilled through its shell - incredibly cruel! - to a string and a peg so that it wouldn't 'wander'.
My brilliant Dad and his brother executed a night-time, commando-like raid into the garden to rescue the tortoise, and then found it a new home. I know they committed theft, but I was (and still am) so proud of them.