Okay, I've driven with sun-blindness before, but highway driving when you can't see the speedometer...
I wasn't actually super-anxious, though -- I definitely slowed down whenever I couldn't see the speedometer (and FCS-Ian would alert me when I got down to 45 or 40 -- the posted speed limit was 55), but there wasn't a lot of traffic (it was ~7:30am on a Saturday); I think with more traffic, the limited visibility would have made me a lot more nervous.
Driving on Mt. Auburn last weekend, I really felt the difference between 20mph (which I'd been comfortable driving at) and 30mph (the posted speed limit), so I was nervous about the ~60mph nature of highway driving, but because of the road surface, I mostly didn't feel it at all -- which is a little nervous-making, because I can't rely on my bodily experience of the driving and have to be sure to keep an eye on the speedometer (and the traffic around me, which would usually be primary, but in the absence of much traffic, very much the speedometer) or else I could easily drive a wildly different speed than I intended, but I was mostly fine.
I didn't do the full drive to Framingham, 'cause we used up FCS-Ian's anxiety quotient (not that I was driving badly!) plus he was helping with AV stuff at Super Saturday so he wanted to get there early-ish and while I was hovering around the speed limit (even getting up to ~65 at times) when the sun wasn't blinding me, obviously he drives faster.
We talked about doing some Sunday morning (pre-church) highway driving (possibly on north-south roads, to avoid the sun-blindness issue) because he wants me to get to practice changing lanes on the highway without there being much traffic.
On the drive home, we had productive and enjoyable church-related conversations, in which I acquired more to-do list items, but I am currently in phase one of my "do laundry then go the fuck to sleep" post-Super Saturday to-do list (I was out late last night because Drag Gospel at Club Cafe, and up early this morning because Framingham; bonus: I biked ~16mi yesterday for the first time in a while [my work commute is ~8mi roundtrip] and my knees were definitely feeling that during the afternoon workshop today).