The Boston Marathon bombings have shown that, in reality, we are not safe to the extent we would like to be. However, how could you possibly make the marathons more secure (keeping in mind practicality)? Are you going to have scans/searches done on all and participants and spectators within the area? That way, we can stop bombs that way. Are you going to ban firearms at the event? -- don't want another Sandy Hook! Are you going to ban spectators altogether? No spectators would be participants would have to be carrying the bomb, and we can just scan/search them before the event. What about banning planes from flying above in case of another 9/11?
Ultimately, there wasn't a practical way to prevent the bombings. And no amount of hysteria or fear will change that.
I think that higher safety measures would greatly benefit the attendees of such events, especially in light of the recent tragedy.
Marathons, for the most part, are planned out ahead of time, continually announced and sometime very-well attended. It's usually pretty well-organized, as far as where the by-standers will be, expected attendance, and what roads be used. Because it's such a public event, anyone who wanted to could easily gather enough information to plan a well-thought out attack.
Of course they are not secure enough. This was proven with the April 15, 2013 bombs set off at the Boston marathon. It is so easy to slip a bomb in anywhere now. People are sick and degraded and think nothing of destruction. We can beef up security all we want, but nothing is ever truly safe.