I understand the desire to eject and focus on the new work, and certainly that there may have been more than one reason to want to make your move in the first place. I work at a university as well, and understand the challenges and difficulties in moving up inside them.
That said, recognize that if you've done what many sysadmins do and hacked together a bunch of tools to make everything hum without excellent documentation (I certainly have) then you owe your replacement (if not your former bosses) some assistance. You are part of institutional memory and even if you documented your solutions well, you likely didn't document the reasons behind them, or the politics that surrounded them. The new guy/gal will need that context.
Give your one-day hand-over, and in it, set a very clear protocol with boundaries that everyone can agree on (including your new bosses) for providing assistance to the new sysadmin. Make it clear that off-hours is out of bounds, or create a process that avoids you hiking across campus on no notice. Questions will arise over the next several years, though they should taper more or less exponentially. Expect it and be prepared. The value to you of not being a dick about it will be enormous, especially if you plan to stay there for a substantial period of your career (and as a fellow uni employee, why would you not?) will be substantial.