I just bought an entry level mountain bike - Trek Marlin 6. Last time I rode a bike was 45 years ago. On the flat with minimal wind against I've been averaging 14km/hr over 12km as that's comfortable for me. I'm definitely a bit heavy and also probably 60% fit but it feels like a disappointing speed for the flat. I also start to get a sore neck after an hour and find it painful to look up and ahead too often. Is all of this normal? What should a normal speed for someone 60 years old over 115kg on that bike?