I’m rebuilding form from a tough winter (work/travel + covid + flu = no volume and easy fatigue), with my A race in July, and important B races in June and September. I’m a strong diesel with good repeatability, and can typically ride competitors off my wheel on long flat or rolling sections. This makes sense, given that I live in the Delta, and its rare for my rides to have much climbing.
But I’m a terrible climber. Once the grade goes much over 5%, I’m getting dropped like a hot potato.
So - the question - the next couple of weekends, I have free time for longer rides. I’m planning to do my scheduled 90 minute threshold workouts, and then add 90+ minutes of Z2/3 to add volume. Am I better off to ride for an hour or two and then do the scheduled workout, or should I knock out the prescribed workout and then JRA for another couple of hours?
Honestly think personal preference trumps any marginal benefit of one approach over the other. At the pro level there seems to be a trend towards doing the interval work at the end, on the basis that’s more specific to how they race (long races/stages where the key decisive moments tend to happen in the final third) and a better predictor of race success. But personally I’m not a pro, the races I do are rarely much longer than 2 hours (and whenever I have raced longer events up to 4+ hours I seem to be relatively stronger at them when comparing to people I also race over shorter distances), and I prefer to just get the work done and then enjoy the rest of my ride without it hanging over me. So that’s what I do!
If you do intervals first you’ll be fresher for them, and so you may be able to do a bit more time or a bit more power or both.
When you finish your intervals you’ll be a bit glycogen-depleted and this may make your subsequent endurance work more effective.
But it’s probably marginal.
I think it would be a good idea to bang out the structured intervals towards the start of your ride so you’re fresher. You’ll probably get more quality out of the intervals doing them first, then you can tack on that extra Z2/3 volume afterward without stressing too much about needing to hit those harder efforts at the end of a long ride.
I can certainly understand the argument for wanting to do the efforts at the end of a longer ride, but doing so can be pretty tough. I think since it sounds like you have some extra time for these longer rides for just the next couple of weekends, it would be better to make sure the structured work you get in is still high-quality, and then get the volume in after that.
I’ve personally tried to do harder efforts at the end of a long ride when I’ve had some extra free time with mixed success – if you’re fresh and well-fueled, hitting them and feeling good makes you feel pretty pro. 
…But I’ve also had instances when I’ve been super cooked and unable to make the most of the efforts. 
Anyway, I think you’d probably get the most out of the intervals by doing them earlier on so you can hit the power you need to. But if you’re looking to experiment, hey, it never hurts to try something new out!
I do my intervals on the front end so they will be high quality. If I save them for after an endurance workout, and can’t really complete them well, I feel like a failure all week. I don’t need that, so I do the hard work first.
If you have been recently knocking down threshold interval work during the week, then I’m of the opinion it’s better to use the weekend to see how you perform when not fresh. And my coach was of the same opinion. So longer weekend rides would often see mid, or mid-and-late, or late interval work. And don’t sweat the details on the endurance work, just ride around as the legs see fit with an eye towards not impacting the upcoming week’s work. My days off are Sunday and Thursday, so my key interval sessions were Monday and Friday. Saturday was about a longer ride, with or without intervals depending on how I felt.