At Tamarack we put a huge emphasis on our horses fitness. The fitter your horse, the easier their job will be and the less likely they will be to injure themselves. It's not, however, just about galloping.
The majority of our conditioning is done at the walk and trot, we frequently do an hour and a half or longer rides, sometimes it is just walking, others it's trotting when the footing is good. We start and end every ride with a good walk. Frequently I will hack before doing flatwork but, if I don't then I walk for at lest 10 minutes in the ring. Every ride is finished by a nice hack as well.
It seems to me that there are many people who don't think you can accomplish much, fitness wise, at the walk. And half of those people are probably right because they probably don't make their horses walk with purpose. Most horses, when left to their own devices will walk slower than molasses, this isn't doing much for their fitness level. Think of yourself walking, if you went on an hour stroll, taking your sweet time, how tiring is that? Not too tiring compared to you going for an hour power walk. It's the same with horses, if you aren't worrying about fitness then by all means, let your horse walk however they want to. But, if you are thinking in terms of fitness and exercise then you are probably going to have to work to get your horse to march forward in the walk.
So many people just say, but my horse doesn't walk fast, and when they go hacking they get left way behind. None of the horses I ride naturally walk fast, but I have taught them to all walk with more purpose. When you are first teaching your horse to walk more forward they will probably jig some, it's not big deal, you just bring them back to walk and ask again. Eventually they will start to get the idea and you will have to ask less and less.
Several years ago I was fortunate to be given the ride on Jetting West, a horse both Denny and Sue Berrill had competed through preliminary. Westy had been off and when we were bringing her back into work, all we did was walk. We have this sandy area we call the bean field that is about a mile around. Denny and I would walk and walk and walk around that field, we gradually built up to this of course. We kept a brisk walking pace, we didn't let them creep around that field. It was shocking to me how muscular and how fit Westy became just from walking! When it came time to start her back in real work she was fit and ready to go, we didn't cut corners and had taken all the time that was needed at the walk.
Westy was my first really experience with getting horses fit from walking. Of course when you move up the levels more trotting and galloping need to be integrated, but don't short change your walking!