Deciding on a route to use for your bench marking, and future time trials can be very simple as you don't need to set a speed or time limit.

The distance doesn't need to be exact but the route should be easily repeatable in that it starts and finishes in the same location. This means for every metre of incline there is a metre of decline and an overall gain of 0m in altitude.

Start and finish near a park or far enough away from home that you can do a good warm up run before hand.