If your body were able to adapt to low G without loss of robustness, then you could never have quality of life on the Earth because your bones, muscles and connective tissue would be too underdeveloped to handle a full G. Imagine if today you went from Earth to a planet with nearly 3 times Earth's gravity. Your impairment would be similar to that of someone who's morbidly obese and wearing a hat made of lead.
I see dim prospects for humans living on Mars long-term. The human body is not designed for 0.38 G.
It’s the radiation. The radiation in space getting there and the radiation on the surface living there. For the living there, colonists could dig tunnels, but has any robot dug a tunnel on Mars yet to see how feasible it is? And if you start sending humans right away there’s no robots to dig tunnels for you so you have to dig them yourself. How long before your tunnels are deep enough and comfy enough to allow you to duck out of radiation-range most of the time? And if you’re going to live in a tunnel, why not do it on earth, where the gravity is more healthy?
The whole idea is stupid to me. We should be going to the moon, setting up tourist destinations in lunar lava tubes, pre-made tunnels. We can get there in days and stay for a short time and come back long before our bones atrophy.
Going to Mars, even if there are tunnels waiting there for you, will expose you to 6 months of deadly solar flare risk and constant bombardment from galactic cosmic rays (for which there is NO adequate shielding outside a deep atmosphere or tunnel).