I ride my single speed ( not fix gear ) about 4kms a day to school and no problems there. On my route it's not full flat, well there are two routes, the longer but flat way or the short and a hill you have to climb and get over it. Hills aren't a big deal for single speeders as long as you have strong legs but before I got into single speeding I use to ride a gear and trust me those things make everything heaps easier and allows you to get from A-B faster.

Single speed / fix gears require less maintenance compared to conventional gear bikes. Gears are good on "extreme" uphill and topping speed on small downhills and flatground. Single speeds don't loose much on the downhills but fix gears do. End of the day it comes down to how big your legs are. I'm going to attempt riding 40 kms with a 5km ascend on a single speed one of these days. These are the times which I wish I rode a geared bike.