RE: Interval training to maintain weight / increase fitness
No, not too much time at all. It all depends what you are doing as intervals of course. But if it were say, basic track work, you would neeed to follow a protocol like this:
Warm up 4 x 400m easy jog (5 mins)
Stretches - A good 5 minutes maybe more.
Intervals (20 mins).
Warm down and stretches.
That is going to take a good 40 minutes. This would be how I would tailor my track sprint sessions, but it would apply to most forms of interval sprints (rowing + cycling etc). Although HIIT seems a quick form of exercise, the strains it puts on the body and the amount of limbering up you should do to keep it safe often means that it is as time consuming as a longer, steady cardio session.