Three hours in the saddle is a long time. If the o.p. saddle can pull that off I'm wondering seriously if that isn't about as good as it gets. Maybe start taking butt breaks before the 3 hour mark. Simply unweighting the saddle for a few crank revolutions every 15 to 30 minutes is something. I see no mention of padded shorts! No one can do those kinds of miles without a good quality chamois. I'm 5'10" as well but I haven't seen 170 since around 1977. I once self measured at around 100mm. The SMP TRK is just too dam wide. I've got two that are free to a good home: the standard (165?) and ultra-wide for the missus (180?).

The width at the sitbones is not the issue. The width much further forward from the sit area has to be very narrow for me. A number of e-bikes are hitting the market with really nice saddles that are super wide and flat at the sit area but narrow rapidly to a narrow point. The newly re-introduced Mongoose Envoy cargo bike has a supremely comfortable example of this kind of saddle. I contacted the parent company of Mongoose to see if I could get some to stash, but there were none as the Envoy had been discontinued. I intend to ask again as I see the Envoy is back in production.

For a more racing oriented saddle, the Bontrager Affinity is very nice. Flat front to tail, but also side to side with a slight groove just like the H1. I've been finding them used and new old stock on Ebay. A lot of good saddles you thought were gone forever can be picked up for cheap on ebay.