OP - i find that seated comfort and contact points to be the biggest factor in riding distance on a fixed. I've not done the mileage you have, but on the shorter rides that i have been doing over the last year, this has been what has held me back.

Some things that have helped me - tilting the road bar slightly higher than normal. Without hoods i found it hard to get a comfortable hand position that would mimic everyones favorite. By tilting the bar slightly higher it "flattened" the section where the bar curves and drops. Now its super comfy. I explore all the nooks and crannies of the bar. There are many positions available.

Butt - B17 I found seats w/cutouts tend to do more harm than good - the edges become pressure points as the padding is compressed over a long ride. As well, a slightly higher position will alleviate pressure from your twigs and berries and place it on the sit bones where you want it. Realized yesterday that a micro adjust seatpost will be my next upgrade so i can dial in the exact tilt I want. Now its either a bit high or a bit low.

As for not being able to stand to relieve pressure, that is a draw back to fixie riding. All i have found is sliding back on the saddle the change the pressure points. Taking a 2 minute rest to stretch hamstrings, groin, quads and calves makes all the difference. So far i have to do this after about 25 miles.

28mm tires definitely make the ride smoother, I'me sure you will love going to 25s.I remember the ride of my Michelin Supercomp HD 23mm, smooth, fast and never flatted, wish they still made 'em. Wore out quick though. Changing a flat on a bolt on hub during a LD ride is not an option for me, so im sticking with the panaracers. smooth enough and they seem to be bomb proof. No flats in NYC for over a year!!!