That's one way to look at it.
However, exercise alone doesn't lose people much if any weight, they also have to consume less calories. As well, if their focus is health they need to consume more nutrients. I mean, the overweight person could change from 3,000kCal a day of doughnuts to 1,500kCal and lose weight, but for optimal health they're better off with 2,000kCal of fruit, vegies, nuts, wholegrains, beans and fish.
Generally speaking, processed food is less friendly to the environment and more wasteful than whole fresh foods. Obviously as it's processed there are losses of energy both in the processing and the food itself, but as well processed food generally has more packaging, and because it can be transported across the world is more likely to be. Your Uncle Toby's muesli bar probably has more miles in it than your bag of oats, your bottle of applesauce more miles than some apples.
Now, as for "fitness". Well, fitness means "the ability to perform a task." If the "task" is health, well you don't need to cycle 140km for your health any more than you need to squat 140kg for your health. If it's just for your health, then a 30-60' walk, or 150-200% of that cycling, well that's plenty. And so there we're talking about a commuting length of ride, rather than a dedicated ride.
As for the resource consumption of it, unless you have a truly top of the line carbon fibre bicycle, it's really not very much - certainly not compared to machine-powered transport. Likewise weights - barbells last a pretty long time, and are 100% recyclable.
So if we're talking about fitness for health, then the food consumption is actually far less wasteful than the typical Western diet, and the resource consumption for the exercise part of it likewise.
If you want to critique the fitness world on the basis of wasteful use of resources, you can say that we only exist because of a massive energy surplus, ie fossil fuels. Historically people ate good food because junk food simply wasn't available, and they were physically active because they had to be to be able to eat. Junk food can only exist because of massive energy surpluses, and people can only be sedentary because of the same.
In the middle ages, 10 out of every 11 people were involved in food production; 10 people were needed to produce enough surplus food to keep 1 other person going, so that person could be a noble, priest, blacksmith or whatever. Various low-resource technologies like literacy and crop rotation took us from 90% farming down to 25-50%, but fossil fuels and their derivatives have taken it down to 2% or less.
Because people can eat crap food, they do, and because they can earn their keep without sweating, they do. Which if they do for a few years leads to their being fat, sick and weak. So the fitness industry only exists because of a massive surplus of energy in the form of fossil fuels. In a century or two it won't exist, or it'll just be someone training a nobleman how to fight, that sort of thing.
But in a century or two there are a lot of jobs which won't exist or will be greatly diminished in importance. Halve our oil consumption and take us down to 1kWh a day of electricity consumption and we won't have as many Personal Trainers, but also not as many Set Intimacy Co-Ordinators, Diversity Managers, Public Relations Consultants, Stockbrokers, IT Support Teams, and so on.
Consider what mobile phones might cost if Solange (see above abc article) and her colleagues had to be paid even US minimum wage. From USD21 pw to 290 or so. Oh and some workplace safety laws in her mine, too. We wouldn't be replacing our mobile phones each year.
