1. Rig the treadmill to a device that will charge a battery. Then you can carry that battery anywhere as a portable power supply.

2. Rig the treadmill to a wheel that winds up a powerful coil spring (like alarm clock spring). You can bring that wound-up spring anywhere, hook it with something that will convert rotational energy to whatever energy and then toinnggg... release the spring's pin to unlock the energy.

The energy conserved by either method is not 100%. You'll lose 30-40% in the storage conversion process I think.