What would short distance training do for my endurance performance? Due to limited time I've been working on short intervals (generally 1-6 minute repetitions) and the longest ride I've done in 4 months is 90 minutes which, while I've been pushing on the climbs, I still get home with a bit to spare.
Still riding 3-4 times a week, and seeing performance gains at these distances, generally measured in seconds and generally because I can maintain a higher pace for longer at the same rpm, i.e. pushing bigger gears for longer.
How might this training affect e.g. an 8 hour hilly ride, where I'll be pushing on but always within sustainable limits?
You can improve metrics like FTP, VO2Max, etc, with shorter workouts, and those improvements will carry through to longer rides. Your body doesn't draw on a completely separate set of capabilities when you go for a long ride.
But at some level, you need to prepare for a long ride by going on long rides. A saddle that is comfortable at 40 miles might be uncomfortable at 80 miles. Likewise for shoes, etc. Small positioning problems get magnified.
Also, your body can generally store enough energy to carry you through about 2½ hours of riding, after which you're at risk of bonking. Going for longer rides gives you practice refueling, figuring out what works and what doesn't. You can also train your body to run on stored fat more efficiently (even if you are very thin, you have enough stored fat to get through days worth of riding).