If you are not eating enough fiber add it into your diet a bit at a time. Do not go out and eat an entire sunchoke casserole.
That is where most people go wrong. You body needs time to adjust to any change in diet.
That depends on the individual. Don't forget, legumes such as lentils, chick peas and a myriad of others. they are easy to incorporate into your diet, but people are unwilling to eat "healthy." Again, it is simply a case of culture.
Consider the Indian diet; it is full of legumes and their national dish is "dal," lentils cooked in innumerable ways, all great and all with flavor and fiber.