time in 1979, sight unseen, and I got to the top of the Heartbreak Hills ... I said to the person running next to me, 'So where are these so-called Heartbreak Hills?'" said Joan Benoit Samuelson, who won that year. "And he looked at me like I was half-crazed. He said, 'Lady, you just passed them.' In my training, I do a lot of hills."

Chicago

Four world records have been set at the Chicago Marathon, which starts and ends in Grant Park. AP Photo/Nam Y. Huh

This October will mark the 40th anniversary of the Bank of America Chicago Marathon, which, like Berlin and London, is flat and fast. Each year, as many as a million spectators line the loop route that begins and ends at Grant Park. More than37,000 runners started in 2016.

History: Four world records have been set here. In 2001, Catherine Ndereba of Kenya set the women's mark of 2:18:47, which was broken the next year by Paula Radcliffe of Great Britain, at 2:17:18. The men's record has been set twice: by Great Britain's Steve Jones in 1984 (2:08:05) and Morocco's Khalid Khannouchi in 1999 (2:05:42).

Key feature: How flat is the course? There's only about a 30-foot difference between the highest and lowest points.

Scenic highlight: Views of the Chicago River winding through the city are plentiful. Runners cross the river five times.

Quotable: "It's an excellent course, great organization and a vibrant city to race in," Radcliffe said.

London

The Virgin Money London Marathon is not only a great opportunity for elite runners to set records --