First off I want to apologize to observer about yesterday . . . I was in a very bad mood.
Ok, you bring up some interesting points. I do recall them amending their offer to require there be some advance notice... but I don't know exactly which (the $100k but not $10k?).
The modern rule is that you can't revoke once there has been some initiation of performance. It can't just be mere preparation to perform (like if he bought some shoes). But yes a jury might agree that training to run that time is a significant detriment taken on that the offerors should have foreseen would happen. If they tried to cancel a second before the race, then the jury might decide it is unfair and that there was a breach - essentially because it was unfair.
Different side of the same coin . . . if he gave them 1 second of advance notice and they don't pay, the jury could say that 1 second was unreasonable because the purpose of the advance notice requirement was clearly to attend/track/video/whatever the performance . . . so they could say 1 day was reasonable. In that case they would have a defense to breach (it isn't enforceable).
Also, telling someone that you will perform isn't usually the same as performance for unilateral contract. You need to make a solid effort to begin the performance - a simple promise is not sufficient.
The real issue here is that they gave him 12 months explicitly. This is where you'd have to do some legal research, because the party claiming breach (rossi) could claim that the offer needed to be held open (can't revoke before). For the UCC/products and goods there is no consideration required. But for services common law applies, and usually some consideration is required. Rossi could say that his training in preperation for an event that would make brojos money was sufficient consideration, but that is debatable.
In other words, the 12 month thing is what would make revocation not cut and dry IMO. A purely personal speculative opinion.