What is a reasonable speed for long distances on a bike?
Speed varies widely by cyclist, depending on fitness, road conditions and traffic.  Some of my observations (cruising speed based on a flat, paved road in good condition):


20km/h (12.4 mph) - many "occasional" cyclists ride around this speed
25km/h (15.5 mph) - most commuters
30km/h (18.6 mph) - fast commuters, slower roadies
35km/h (21.7 mph) - fast roadies
any faster than that on a long flat and they're probably a racer


(based on who I pass and who passes me when riding around 30km/h)

Average speed will usually be slower than you think, once traffic stops and hills are factored in, especially over longer distances (like 80km).  On my 21km commute I'll hit 30+ on every long stretch I can, but my average still only works out to 24km/h.  For longer rides I cruise around 27-28 km/h, which is more sustainable; averaging 22-24 over a very long ride (200km) is a great pace for me.