I think I agree, on reflection, that it is not a binding contract. But (correct me if I'm wrong) I don't think it's a matter of consideration- in a wager, each party stands to gain the agreed upon amount depending on the outcome, which should qualify for that criteria AFAICT. I think the informality of a between-friends bet and gambling laws are what do the example in.
Yeah that clarification helps and I agree there - there's a bunch of exceptions of 'joking bets' etc. I first read the offer as "I'll give you $100 if you can run there and back in under a minute" rather than each party being able to win the $100 which would indeed satisfy the consideration requirement.