Building up walking in a lurcher puppy

Didn't know where this would best fit, but given that it may affect long term health thought I'd try it on the health forum

I've seen so many warnings about not ruining a dogs growth/bone/muscle development by over exercising them as a puppy. And loose rules like "5 minutes for every month old". I'm now worrying about what to do! Could anyone give me more info on this & share their methods/experience?

For the record my pup is a 14wk old 4.5kg lurcher. We have a field about a ten minute walk away where we like to take her. But 10 minutes there + 10 minutes back = 20 minutes :s so is that already too long without even walking around the field (off lead)?