In hilly terrain, where the distance of the uphill portions might equal the distances of the downhill portions the average trip speed can never exceed twice the average climbing velocity. Imagine riding an upright cruiser, grinding up a long steep hill 1 mile in length, and needing 30 grueling minutes to complete the ascent. The climbing speed was thus 2 miles per hour. Now add to the vision the bike's handlebars being equipped with a near-instantious teleporter button. Thus the total round trip of 2 miles would have happened after 30 minutes climbing plus the infinately small quanta of time needed to go back down hill.
Total time ca.
Total distance 2 miles. Eschewing the mythical teleporter, if the descent were done with a cautious application of coaster brakes, say at 12 mph, the downhill leg would grow to 5 minutes. Total trip time would then be 35 minutes. A 0.5 mph difference. Significant if one is fighting to win the Tour de France.