The most fun thing is to go on hilly group rides with people who are a little faster than you are. Then you get a little of everything. A 3-4 hour ride provides a good bit of training stimulus. So that's one way to do it. The other of course is a training schedule where you only do one specific thing on a ride, but probably do 3-4 different things, each on a different ride each week. Both training modalities work.

If you're doing the specific objective modality, I wouldn't recommend doing more than one training objective per ride. If you're doing it right, you should be busted after finishing one objective. Doing something else after that would mean not being able to do it well, and thus contraindicated.

If you're doing 10 hill repeats, you're not doing them nearly hard enough. They have to be really hard to get the benefit. The "hard" can be either length or effort. 4 is usually enough for me, sometimes I'll do 5. At 6 I have the feeling that I would have done better to have done fewer harder ones. When you do the last one, you should not be bored, rather on the edge of not being able to do it at all. Say you're doing 6' hill repeats. You ride the hill as hard as you can and not slow down for the whole 6 minutes. You note how far up the hill you got, ride back down and do it again. You should be able to get as far up the hill maybe 3-4 times, but then the next time you don't get as far in 6'. That's when you quit because there's no point in doing slow hill repeats.

If you can't do group rides, just long solo hilly rides are good, too. You should go hard enough that you have trouble finishing the ride if you're after that sort of training stimulus. I basically do two types of rides: hard and endurance. Hard means anything where you are working hard enough that you lose form, speed, ability, etc., until you are just trying to survive until the end of the ride. Endurance means an all day pace where you gear down for hills and try to ride the whole thing without breathing hard. I spend a lot more time on endurance than on hard. The usual recommended balance is 80/20. Try to ride as little as possible in between endurance and hard. It's a waste of energy.