I'm with the majority here, the other factors (especially wind) make it unrealistic to attempt a precise calculation. However, my own figures bear out what valygrl has posted. Fully loaded (c.50lbs of gear) I'd expect to average about 13mph on the flat in the absence of serious headwinds. On a recent days touring, with that sort of load, I averaged 11.6 mph (moving speed, excluding stops) for an 83 mile day which included 4150 feet of climbing. That implies that every 100 feet of ascent added about 1 minute to my ride time.

Interesting way of looking at it, I hadn't thought of doing so before. Your mileage will most definitely vary, though.