@Rockhead,
If all you plan to use on these lines is a computer, you should be fine. Any consideration to running some telephone lines through the same conduit so you have a more reliable telephone resource than the sporadic cell service?
I don't know how the cold weather will affect the CAT5. Will the conduit have enough insulation for the wires?
Depending on the distance, you may also need a signal booster. Back in the day (the 1990's) when I was running cables in the ceiling of the new building our department was moving into, I had to place the hub in a specifically central location of all the cable runs so it could also act as a signal booster.
I'm sure things have improved a great deal since then so one of our other network gurus will probably have better advice.
What about running some romex cable for any future electrical needs in the four areas?