Making some further generalizations . . .
Average speed over a long ride depends on one's ability to recover. Because wind resistance increases as the square of the speed, average speed is most easily increased by increasing climbing speed. Because long rides will have multiple climbs, average climbing speed is also governed by the ability to recover between climbs. So the first thing to work on is increasing climbing speed.
On your 900' climb, the highest HR you can sustain for the last 600' without blowing up will be your functional HR or LTHR. Push it, so you know right where blow-up is. Base your zones off that. You don't really need to do a test since you have that hill to test yourself on. Next of course is to work on recovery, meaning the ability to recover between zone 4 efforts while remaining in zone 3. That's how you work your aerobic system. During a century or other long ride, you'll want to climb at ~95% or so of your LTHR, which is also where you'll want to do your training climbs to maximize recovery.
Next is strength. I do three things for strength:
High cadence drills, meaning to hold a cadence of 115 or better while staying in zone 2, meaning low resistance. Continuously, starting at 15 minutes and increasing to 45 minutes over a month, then repeat.
One-legged pedaling. 2 minutes each leg at 50-55 cadence, 2 minutes legs together zone 2 90-95 cadence, 2 minutes OLP 80-85 cadence at much lower resistance. Repeat for time as above. The low cadence gearing about the same as the legs together gearing. If you cry for your mommy, you're doing it right.
Tempo drills. 70 cadence in upper zone 3. 15-30 minute intervals with 15 minute zone 2 recovery between. When you can do an hour of Tempo at the same resistance without substantial HR drift, you are in good form. Be careful not to overreach when doing this. It's quite a strain. Once a week for 3 weeks, then a week or 2 off.
Muscle tension intervals. 10 minute intervals of steady hill climbing at 50-55 cadence, upper zone 3 - lower zone 4. Recover during descent then repeat. 2-4 intervals.
You can also do speed work on the flat: ride at 100 cadence in upper zone 3 for 1/2 hour continuous, 5 minute recovery, then repeat. Very important not to let off that cadence, so flat or small rollers. Oddly, that seems to help my climbing quite a bit.
When you do your century . . . You've probably heard people advocate to start slow. Don't do that. Go out fairly hard but not to the point of leg pain. You'll probably pass a lot of people. That's the idea. After 15-30 minutes, ease off and wait for a group of riders to go by about 2 mph faster than you're going. Latch on. After you go hard on the first couple of hills, things will get sorted out and you'll have good people to ride with.