I do agree that the knees take a beating. A few weeks ago I did 115 miles fixed. Here in Iowa we have a great trail system. In my area there are over 600 miles of paved trails most of them being old rail road tracks converted to bike trails. Because they were train tracks at one time the grade can't exceed something like 4%. So these are great for fixed long distances training rides. The only traffic you have to worry about are crossing gravel roads out in the farm lands. Every 6-12 miels on my 115 rout hits a small town. But even with only 4% grade at the end my knees were feeling it for sure. Great way to work out the winter rust from your legs!

There are actually two speeds on fixed gear. The first is pedaling and the second gear is pushing your bike. There is no shame in pushing your bike if you were packing lots of gear. I generally start doing the snake method by turning the path into switchbacks to make it up.

Momentum is the key. Once you lose that it hurts (more).