Here's a profile of the of the Olympic cycling road race course. Click on the image to view it full-size. The men's race is 245.4 km (152.5 miles) and the women's is 126.4km (78.5 miles).
The course is an absolute killer, one of the most difficult in modern Olympic history. After the first relatively flat 80 km or so, riders in the men's race do seven laps around a hilly 23.8 km circuit, meaning about 170 km of rugged up and down before they see the finish in the men's race. The women follow the same challenging course, but with fewer laps around the circuit portion.