Your odds are way too high. If they were even close we would have hundreds and probably thousands of deaths by lightning every year. Without specific data on the size of the risk pool and number of cyclists deaths it is impossible to determine the actual probability; however, the following numbers do indicate that the 1:250,000 to 1:750,000 range that I quoted from another source is likely pretty close.

Here are some actual facts:

1. Lightning kills an average of 58 people per year source:http://www.lightningsafety.noaa.gov/overview.htm
2. About 700 cyclists die annually, of which about 80% are a result of a collision with a car. source: http://bicycleuniverse.info/transpo/almanac-safety.html
3. I can find no statistics concerning cyclists struck by lightning; however a google search turned up 170,000 hits. On the first page of results there are only five deaths listed, three in 2009 and two in 2006. This would indicate that the number of cyclists killed per year may well be less than 1 or 2 out of the 58 average. Many of the links points to stories of cyclists struck who survived... And one of the deaths cited earlier was one cyclist in a larger group of cyclists.

Clearly, you are at a greater danger from cars than lightning. Yes, you should seek shelter, but do so in a safe riding manner...

For those interested here is some information on what is the safe course to take in different circumstances by the national weather service.

http://www.lightningsafety.noaa.gov/outdoors.htm