It's not too bad but you do have to go up at least 2 flights of stairs and walk a few longish corridors (but better much shorter than some other places).
At any rate, if the OP is coming from a Schengen country, I'd venture:
10 minutes gate to rail platform
25 minutes to Mitte (by S7)
10 minutes to Lush
10 minutes back
25 minutes to VIE
15 min to gate
That's 1:35 without any waits and assuming one doesn't miss the train. Could easily add 1 hour to the above if the train is missed at VIE and Mitte. Doable but tight. If you want to do it, have the EUR coins ready to buy the S7 ticket, and be aware of train times.