I tried to use the 200 bus Christine suggested to get to Cannes - but the driver said that the buses were only going as far as Golfe Juan (despite saying Cannes on the front), as the road into Cannes was closed.
The only other option was to take the 200 bus to the airport, then get the express bus from the airport to Cannes. This costs €17.50 each way!
The organisation of this was appalling. There was no information - Christine has provided more information on here than the organisers, or the bus company provided anywhere.
I felt sorry for the bus driver too - he was getting it in the neck from angry passengers. It was not impossible to drive into Cannes, the express bus (and other local buses) were doing so, so why couldn't the 200 take a different route into Cannes?
Failing that, given the circumstances the organisers could have put on some buses for supporters as well as runners. They could have charged, people would have been happy to pay a reasonable amount.