The main walkway of the Zoo, the Olmsted Walk, is about 0.9mi from one end of the Zoo to the other. However, if you plan spending the day visiting the different animals expect to walk over 3mi.
The Zoo gets really crowded and hot in the summer. The best times to visit are the Spring and Fall.
You should be able to see everything within a day. If you are strapped for time, spend some time before your visit figuring out what exactly you want to see. The website is a good resource for information. Check it out before visiting.
A few things we highly recommend:
1. The Pandas - on loan from China, don't miss your opportunity to see Tien Tien and Mei Xiang.
2. The Great Apes - if you are lucky you will get to watch the organgutans travel along the o-line.
3. The American Trail - observe sea lions and seals on this new addition to the zoo.
If you have children, take them to the Kids' Farm on the far end of the Zoo to pet various farm animals.