"Not at your age !"
"A young person with physical fitness may have the required tolerance but absolutely not you." ?????
Oh MadManascar come on !!!! You know, I've enjoyed reading your postings, and before you went to Madagascar your questions addressed some really interesting points, but this is ageism of the worst kind. How on earth do you know that these people "absolutely" do not have the required tolerance? Why in the name of any deity you may happen to believe in do you assume that someone of their age can't handle taxi brousse trips?
At the risk of completely destroying whatever image I might have on this forum, you might like to know that I'm over sixty myself, that I frequently travel by taxi brousse when I'm in Madagascar, and certainly not just on the blacktops, and that I often enough out-"stand" and out-"tolerate" twenty somethings who are completely shattered by the experience and climb out of the vehicle swearing never to darken the door of a taxi brousse again. Correspondingly, I find it absolutely hilarious when I read a posting by some guy who is presumably three decades or so younger than I am telling my contemporaries that they (and by implication I) are past it!
Sure, physical fitness helps - I can still trek the Masoala Peninsula. How do you know that Dave and his wife aren't physically fit?
Dave, the description MadManascar gives of taxi brousse travel is accurate in most cases - time-served vehicles packed with more passengers than they should have, uncomfortable seating, badly maintained suspension, bumpy roads and long journey times. Only you know whether your implied decrepitude has advanced to the stage where you can't handle that. The mere fact that you're in your sixties has by itself nothing to do with it - I know that, which MadManascar, much as I respect the guy, doesn't and cannot (yet!).
If you do feel a bit apprehensive about this kind of travel, there are one or two ways to make it easier. Between Tana and Mahajanga, which is the route you would take for Ankarafantsika, there are full-sized Mercedes buses in addition to the ubiquitous minibuses. These are not overcrowded, though as they seem to be ex-German army, they're not luxurious either.
Most taxis brousses on the tarred roads are Japanese minibuses. You can travel more comfortably if you book the seats next to the driver (warning: the Malagasy call these "the seats of death"!), or, slightly safer, seats in the row behind the driver. There's usually more legroom there. To get these, you may either have to book a day in advance or choose a vehicle that hasn't yet begun to fill up, which will mean waiting longer before you leave.
Hiring a 4x4 is comfortable and gives you flexibility, but it's also expensive, and you miss out on the contact with the people. Leave that sort of thing for youngsters with a soft upbringing and less tolerance...(!)
I wish you well, whatever you decide to do - Madagascar is a fantastic country.