If you want to do this i would suggest:

- Flip flop hub

- 2 brakes, front and rear

- Gear the fixed side low for flats and climbs

- Use freewheel on descents

I tour on a geared bike, but commute fixed, and seattle ain't flat. You get used to the hills, but it's always the downhill that tires me out.

I've had a pannier grocery load of about 30 pounds, and front and rear brakes really helped slowing down coming down the hills, highly recommended.

What I like about riding fixed is that it forces me to push myself harder than i would on a geared bike. I power up hills and take it slow on flats, it's a hell of a workout, but I don't think I could do it day after day on a tour with a load.