Comes down to this . . .
I had a similar experience and tried many low to mid-range saddles which claimed to be ergonomic and only provided moderate comfort over maybe the first 10 to 15 miles. I could tell you long stories and complex theories as to why this is, but it comes down to this.
Get a real saddle! For the distances your riding--I'm a marathon rider myself--you should be on a serious racing saddle.
Seems to defy logic, but one of those harder, leaner, meaner saddles, when properly adjusted and especially after breaking it in with a couple of hundred miles is MUCH better and more comfortable than some saddle made for comfort. (Comfort saddles are actually not engineered for 35 mile rides--they're for 5 to 10 mile rides, after which the 'comfort' material compresses and after many rides breaks down slowly but surely).
The 'love canal' is a healthy choice, but comfort is largely a matter of proper fit and adjustment not only of the saddle, but the whole bike down to what materials you're using.
If you really want the saddle you can stay on for hours then settle for nothing less than a leather saddle--break it in just like leather shoes. I would also suggest to get it with titanium rails but realize that's very pricing. Don't get steel rails though--go with cro-moly at least. I believe the extra cost is well worth it because the saddle will probably outlast the bike if properly cared for.
Changing to a better seatpost, such as a Thompson Elite, also helps greatly.
My butt is rather happy with my Bontrager Race Lite saddle with a gel dot--I have one that's leather with titanium rails and one that's synthetic with cro-moly rails (which I bought on sale and is the same design aside from the materials) on my two bikes. Both are hand stitched in Italy by Selle San Marco. I was never able to ride for hours at a time until I bought my first real racing saddle.