terrorism is a relatively new thing here, but being from the UK and particularly having been in the armed services I lived under the concern/threat of terrorist attacks for about a decade or more... looking under the car before getting in, checking the mirror for tails, taking an advantageous seat at restaurants (a habit i cannot avoid even 20 years later), and being very quick about Christmas shopping...all prime targets in the day.. the threat is ever present, it is real, and attacks will happen.
All i can say is that terrorism operates by fear, and it is up to the individual to determine if they will let that change their lives..
Regardless, no matter what the authorities may say, there is no set of security precautions that can protect a 26 ml road race ran by thousands and viewed by tens of thousands, and with an assumed level of blanket security I suspect that the whole experience will not be the same for many years... so if it was me..i wouldnt bother..but for the latter reason, not the threat level.