By Jim Melwert

PHILADELPHIA (CBS) – It’s Marathon weekend in Philadelphia, with tens of thousands of runners taking on the 26.2 miles of city street.

It starts with a race expo Friday afternoon at the Convention Center, an 8K and kids fun run Saturday, then wraps up Sunday with the half and the full marathon, now in its 21st year.

“So in 1994, imagine this — race weekend welcomed a combined 1,500 runners,” race director Desiree Peterkin-Bell tells KYW Newsradio. “Today, over the course of our race weekend, we are going to welcome about 30,000 participants.”

She adds the course features some of the best scenery in the city:

“You start in front of the Philadelphia Museum of Art, head down the Parkway into Center City, run through Old City and you see some of the historic sites like Independence Mall and the Liberty Bell.”

And that’s just the first five miles. The course also goes past the Philadelphia Zoo, Kelly Drive, and Manayunk.

Peterkin-Bell says 16,000 runners will be going the full 26.2 miles. Among them is Bob Koen who is one of only two people who’ve finished each of the 20 previous Philly Marathons. His advice to first-timers: beware the hills:

“Especially the last 12 miles, so people shouldn’t go out to fast. Try to have a negative split where you run the second half faster than the first half.”

New this year — a finisher festival with food and family fun. There are street closures, transit detours, and security restrictions in place.

Complete Guide To The 2014 Philadelphia Marathon

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