I've done it a bunch of times. I found The Shubert to be the most difficult view, as the ledge was tall (and I'm short), so I couldn't really see. This was during Spamalot, and I don't remember if there have been any renovations in the past decade, so it might be different.
I get distracted easily, and depending on the theatre, the ushers may be sitting/standing in the back of the theatre (as well as stage crew on occasion, when actors enter/exit through the aisle). When I did SRO for Hamilton last year, an usher was eating on the staircase during the show, which I found distracting.
I'd recommend not getting to the theatre too early. There's a lot going on with seating, and SRO can get in the way. Take advantage of the chairs in the basement by the bar/bathrooms (if they have) before the show and during intermission, if standing is a lot for you. In general, it's not that bad. I find that if I'm not enjoying a show, I notice my feet getting tired, but if I'm engaged in the show, I forget that I'm standing.