Connecting a resistor across the terminals of a motor used as a generator will give you a braking torque proportional to speed, so-called dynamic braking.
Control the constant of proportionality by changing the resistors. Open circuit / high value resistor -> no or little braking torque, short circuit / low value resistor -> maximum or high braking torque, mid-range resistor -> somewhere between these limits.
The mechanical input power to the motor/generator will be dissipated in the total resistance of the motor + load resistor circuit. As an exercise machine could produce 1 kW peak, and half of that continuously, you need the load resistor power-rated accordingly. If you go for the maximum torque of no external resistance, that means all the power will be dissipated in the motor, which needs to be cooled appropriately.
Many exercise machines use an automotive alternator as the load generator. It has a number of significant advantages.
- It is mass-produced, so new ones are cheap. It's thrown away in large numbers, so secondhand/rescued ones are free.
- At several kW, the power rating is more than adequate.
- The load torque can be controlled by using a fixed resistive load on the output and varying the field excitation current, this tends to be cheaper and easier than switching or controlling resistors.
- If you do want to recover the electrical output, then the output voltage can be controlled by the field excitation current, just as it is in a car, without further power electronics.