Walkbehind hands down. They are a much more versatile mower. You can mow plenty fast and get plenty done with a walkbehind. I ran walkbehinds exclusively for MANY years. The only reason I have a ZTR is for my own personal yard which is a behemoth and a few commercial properties and a couple football fields. Other than that, a walkbehind is the way to start out my friend. Just put a sulky on her.
On the getting worn out part, I could argue that a rider would put more abuse on you than a walkbehind. Mowing, whether it be ZTR or walkbehind is the most abusive part of lawn maintenance in my opinion. I'd rather walk 25 miles with a backpack blower and a trimmer than mow for 9 solid hours riding a mower. But, a walkbehind with a sulky should be very efficient for you.