More than 22,000 people will take to the streets Sunday to participate in the Chevron Houston Marathon, Aramco Half Marathon and AAB 5K races. Most of the out-of-town runners are arriving in the Bayou City today. Packet pickup for all the races started Friday morning at the George R. Brown Convention Center.Officials from the marathon also spoke Friday morning on the possibility of rain on Sunday. Chevron Houston Marathon Director Brant Kotch said that because there are Houston police officers working the event who also work night shifts, the course needs to be clear by 3:15pm. So the race could feasibly be delayed for two hours and they could still run the full course. There is also the possibility of condensing the course. Kotch said if there is a delay during the race, runners would be sent to designated shelters to wait for a METRO bus back to the GRB. Some of those safety sites include the Galleria, parking garages and underpasses. Houston police would be giving directions, stopping runners and asking them to wait until the race could resume or be canceled. They acknowledge that bussing people back would not be quick, but would be preferred to letting people run in bad weather. Marathon officials are putting together a list and map of designated safety spots on the route that runners could check it out in advance of the race and they say they will post it online by noon Saturday. Houston Marathon Managing Director Steven Karpas also told us that a couple of the elite runners who have dropped out have done so because they could not secure visas, not because of the weather. Click on this link for complete coverage of the Chevron Houston Marathon. And don't forget to watch the race starting at 7am Sunday on ABC13 and streaming live here on abc13.com.