I suppose it all depends on what you read, but even serious historians have been confused about the life span question. I can't point you at any written sources, but common sense (sorry, Mr Rees Mogg) can tell you that infant mortality figures were far higher, childhood diseases more likely to result in death and the statistics for women dying in childbirth were much worse in the days before antibiotics and the understanding we now have about infectious diseases and the importance of clean water. As indeed they are in countries today without those things. I do know that when I was working on retirement ages, many years back as a civil servant, I was introduced to the concept that average statistics were highly misleading when they weren't adjusted to take these factors into account - I also know that many of the writers and important figures that I have been examining in detail in the 12th and 13th centuries were over 75 when they died, and this wasn't commented on as unusual in any way. Queen Aliénor of Aquitaine was 82 when she died, and had been travelling around Europe until just a few years before this, and again, her age wasn't mentioned as anything extraordinary.
Sleeping sitting up - my husband works in a living history museum where it is always 1645, and they point this out. I will check with him what the documentary evidence is, but they do careful research and I doubt if they would mention it without a factual basis. The carvings on bedheads often start surprisingly high, which is because pillows were placed against them up to a greater height than we do today so that people could indeed sleep sitting. This would mostly be people of a higher income and social status, but then again it tends to be their furniture which survives. At the museum they mention a superstition that the Angel of Death will only take people who are found lying down, so sleeping sitting up will fool death!
As to the height - there are plenty of descriptions of people found in historic documents in which their height is mentioned. In the case of where my husband works, they know how tall the owner of the house was (and he was over 6 ft). Mostly we look at evidence such as clothing and armour which has survived, and this is often from children and therefore has survived precisely because it hasn't been worn out (it's been discarded or preserved because the child has grown). The armour of Henry VIII is often mentioned, but again, this was made for when he was an adolescent.
I'm not an expert in all of this, but there is a great tendency for people to generalise on the basis of very little actual knowledge. We know that improved nutrition has made some difference to height overall, and, as I said, improved medical knowledge has helped life span, but without the full statistics from the past we can't be sure just how much difference we have made.
As to travel, as I've said above - of course some people didn't travel, just as some don't travel much today either. And of course it all took longer. But royal courts did not have just one place to settle and in the medieval period were constantly on the move, along with their large number of servants and courtiers. Merchants had to travel. Farmers and drovers had to get their crops and animals to markets. People went off to join armies, to sail ships, to seek their fortunes. Most people needed to work, and to travel to find a position. There were pilgrimages, which were a serious business both economically and spiritually.
Now, don't get me started on the situation of women in the middle ages - that, too, has been substantially revised over the past few decades when people stopped re-quoting past historians and started re-evaluating contemporary textual evidence and other facts. There's often a great confusion between European women in the medieval period and women in the Renaissance and later, when the church had a more controlling influence.