I'm not riding the southern tier nor have I come across goat head thorns. I do live in a high-alpine desert, but you have to look very hard to find anything cactus-like out here (and there is only one spot where I know they are). Most of my touring (so far) has been in Europe...and like my OP indicated, I will be in Wales in just over a month.

I did an 18-mile, 1800ft elevation gain, ride today. It was very hot - in the 80's here - and I noticed a few things. I am riding with my panniers on along with my handlebar bag, but only carrying raingear in the panniers right now (phone/wallet/snack in the handlebar bag). Just to get used to the aerodynamics and handling with such things on. I got passed towards the bottom of a long downhill by a guy on a road bike - but that was more due to not having the gears to match him (plus the added weight of my bike + rolling resistance). Then I got passed where the bottom of that hill turns steeply up for a short but killer climb. 5 roadies passed me (one got off and walked, which is where I caught up and passed him) but based on perceived cadence, I certainly seem to be putting in more effort to get up the hill. Again, all the factors of riding a Surly Disc Trucker with Marathon Plus tires all comes into factor. I imagine just one of my tires weighs more than both tires on a road bike.

I will go ahead and order those tires - the hard-shell gatorskins - since they also only look to weigh 1/3rd of what a Marathon Plus does. They should arrive early this coming week, which gives me 3.5 weeks to ride them and break them in. It will be interesting to ride the same routes and compare my times - see if there is any significant change.