Is it possible to be in between genders long term?
Yes, it is possible to be in-between the two "standard" genders long term. It is not very feasible in most societies, particularly Western ones, but it can be done. A person will most likely find it very difficult to truly live in a third or other gender, particularly in societies that associate biological sex with gender and legally define a person as one or the other (such as in the United States.)
As for the extra information on the discussion page, that is a tricky question. Gender is typically held to mean a person's internal identity, whether that matches their biological sex, the opposite sex, or some other thing altogether. Sex would be your biological elements such as genitalia and sex chromosomes. Using those two definitions, the question seems more geared towards sex than gender, although gender identity may come into play as well. There are plenty of people who go all the way up to, but just short of, the MtF genital surgery. Finding a set of doctors who will accept and understand a person who desires to have their body as such will be a difficult matter.
Sex and gender are two facets of humanity that are still poorly understood and vastly underestimated in the sheer variety of combinations and expressions that can come from them. Most of society, including the scientific community, still believes that there are two sexes, two genders that match them, and three sexual orientations. While there are truly more than two genders/sexes and three sexual orientations, trying to get other people to understand and accept them can be very difficult, which can make it very hard for a person to truly express themselves for who they are. This is not trying to discourage anyone, though, but merely to inform and try to help.
My personal opinion. I have a real problem with transgender. I don't believe it exists. I think a lot of people use this as an excuse to not admit to being Gay or Lesbian. That said, I think it may take some people longer to adjust to the idea that they are not what they thought they were or what people expected them to be. At some point, you will come to terms with it and go on from there. I never had any issue with it, I knew at 6 what my nature was. Some people figure it out at a later time.