It does depend on the nature and location of the travel.
When travel involves staying at a hotel it is not too difficult to choose something with some options for fitness. When staying with family it can be harder, both to find local resources, and to carve out time that matches one’s own schedule and the schedules of those one is visiting.
I would agree with most of the respondents that maintaining healthy eating can be a challenge. Portion size is an issue with a lot of places, getting to know the lay of the land in a new place can make it harder to find the non chain local venues that will be healthier…. and not all hotels have small refrigerators that make keeping leftovers or snacks convenient. Then airports make it difficult to bring much food, and the food available in them is also challenging. And hotels that offer buffets free for breakfast can create challenges in amounts one eats. I generally try to stick with a boiled egg and a banana with my tea.
The key is advance planning, both in terms of what one does and one can bring, and in terms of locating options on the ground, and indeed in picking the specific locations where one will stay.