I started biking to work every day (approximately 6 miles one-way), and every morning it is a breeze, it takes about 15 - 20 minutes.  On the way back home, it is near impossible, and takes 45 - 60 minutes.  bikely's got this awesome tool that will show you the elevation for your route.  According to the graph I climb 134 ft on the way to work, and descend 594 ft, but on the way home it is reversed and boy do I feel it.

I bought my bike out of my work's classifieds for $30 USD and in the first 2 weeks of biking to and from work, I have had to do a number of things to my bike.   I didn't realize it for the longest time but my back tire had a slow leak and was sitting at about 20 psi, which was no big deal on the way to work, but made it pretty difficult to get home.  My tires are now both around 55 - 60 psi.   I also spent a good couple of hours learning about derailleurs, cogs, chainrings, sprockets, etc ....  My bike would jump gears, slip gears, fail to shift at all, it was a mess.  But I love it now - free exercise and I don't have to deal with too much traffic.

Here's my bike -

It is a GT Timberline (8-10 years old), all tuned up now and looking good, I have definitely gotten my money's worth.