I used to ride lots of centuries on multi-geared bikes. I'm not quite ready for that on my fixed gear, but I did ride 51 miles today.

According to GPS, minimum elevation was around 40 feet above sea level, max was 480. GPS doesn't tell me, but I estimate steepest grade up was between 6% to 8%. Obviously my speeds varied a lot with hills, but on the flats I held steady around 17MPH.

I'm pretty sure I'll manage a century before the end of this summer or fall.

Instead of my usual urban riding, I'd like to try riding from Portland to Seaside Oregon. The most direct automobile route is Hwy 26 most of the way, which winds around a lot and has one summit around 1600 feet.

I'm still a flat-lander at heart -- so steep hills still worry me. But is this ride feasible on a FG?

It's doable. I just had my longest FG ride of about 70 miles the other day. The heat/humidity did me in, but being FG wasn't a bother. Elevation changes are obviously the key, and we don't have any I can't handle.
Sounds like yours isn't too extreme either, though it's all relative to your gearing.

I did a spot-on 60 mile long suburb-to-woah-where-the-hell-am-I-rural-place to city to suburb fixed today, I have tried it 3 or 4 times with mixed results (ridiculous humidity leading to dehydration, walking up 1 or 2 hills, getting lost, etc) but I nailed it today. I did it in just about 4 hours giving a spot on 15 mph average. Headed out to the rural place is a literal climb, so I'd say I only managed 12 mph average at best. The worst hill (Waterloo Rd in Devon, for any people in the PA Main Line) was about 10 percent. The rural area to the city is a straight, level bike path. I averaged 18 mph with 3 24 mph sprints.

My point is doing long rides fixed with the right gear (saddle/hip bag, at least 1 bottle, a good route with multiple libraries/supermarkets/etc for water refills) is key to working up to a century and staying healthy in this almost-August heat.

Don't think I'll be doing a FG century on a very tight Track Pro very soon, at least not in the heat. Today broke me down pretty well and it will take probably a dozen more rides until I feel comfortable with my planned century out to Lancaster. My ultimate goal is to do the Philly to Pittsburgh "race" next year.

I rode from Long Beach to San Diego the other day. 100 miles exactly.Thought it was a little weird when at one point we noticed we were the only cyclists on the road, then we hit the hills. We went the wrong way,inland, and had to ride through canyon after canyon. Oops.

I was lurking the touring forum and read that one dude did a u.s. cross country on a fixed gear.

of course..not a 'day-ride'

I did a 96 mile ride fixed once. There was virtually no elevation change. Having major elevation change is a double whammy for riding fixed because you must not only climb hills on a gear to high, but you are also forced to pedal long distances on a gear too low.

drove that exact route a couple months ago, its do-able on a fg, there is one rather large hill that MIGHT require a bit of hiking, but otherwise I think you should be ok. The descents are what will really tire you out, but I dont think there were many really rough ones there.
hit up the japanese restaurant in seaside, I'm pretty sure its the only thing worth seeing there.

i just started riding ld fixed, but lots of people ride looong distances.
bmb, pbp and the 508 have all been done fixed.
the boston brevet series was done by several riders. 220k, 300k, 400k, 600k - with steep terrain.

kent p tours fixed.

here's my first century fixed - dirt and rollers.