"Sounds good? Sign up for our private beta below and receive 20% discount when we go public"

So you're charging me to have my friends encourage me to work out? I don't understand what I'd be paying for. I could simply ask my friends to encourage me.

That said, I don't think it's a bad idea, but a subscription might not be the right way to monetize it. Gyms might have affiliate programs of some kind, or be willing to pay for leads.

One more thought, a free iphone/android app that lets people easily and quickly let you know they worked out would probably be good marketing.