The average person walks three miles an hour (4 1/2 feet/second), and one can easily stroll along at two miles an hour (3 feet/sec) or even slower. 100 yards = 300' = 66 seconds at 3 mph, or 100 seconds at 2 mph. That is already 1 minute 40 seconds, so I think the 100 yard estimate is in fact confirmed by the reporter (and it would be interesting to see whether she was trying to prove or disprove it).
Also... I know the security is generally tight getting into the stadium. They go through purses, bags, etc. I'm thinking he was maybe trying to find a less obvious place to arrange the bomb to pass security, or something along those lines.