Dense cities are way more efficient than living spread out on the countryside.
Infrastructure cost is lower per person, because higher density means less km of pipes, wires, roads etc. per person. Mass transit is also more efficient in both monetary and resource cost than cars, but it is only viable with the density of a city.

How far away is the average person's house from the workplace, or the market, or the hospital?

I live 3 min by foot from the grocery store. Medium/high rise buildings and mixed zoning make that possible. Idk if you are thinking of American style suburbia, which is indeed very inefficient.

mega cities requires a lot of land elsewhere to sustain those people with the added transportation costs

The farmland needed should be the same either way, but centralising stuff usually makes it more efficient. For example a cities grain needs can be met with a single freight train, which should use less energy than the same amount of grain transported in many small trucks to smaller towns